trunk/oopsePaper/oopsePaper.tex

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\title{{\sc oopse}: An Object-Oriented Parallel Simulation
Engine for Molecular Dynamics}

\author{Matthew A. Meineke, Charles F. Vardeman II, Teng Lin,\\
 Christopher J. Fennell and J. Daniel Gezelter\\
Department of Chemistry and Biochemistry\\
University of Notre Dame\\
Notre Dame, Indiana 46556}

\date{\today}
\maketitle

\begin{abstract}
{\sc oopse} is a new molecular dynamics simulation program which is
capable of efficiently integrating equations of motion for atom types
with orientational degrees of freedom (e.g. ``sticky'' atoms and point
dipoles).  Transition metals can also be simulated using the embedded
atom method ({\sc eam}) potential included in the code.  Parallel
simulations are carried out using the force-based decomposition
method.  Simulations are specified using a very simple C-based
meta-data language.  A number of advanced integrators are included,
and the basic integrator for orientational dynamics provides
substantial improvements over older quaternion-based schemes.
\end{abstract}

\section{\label{sec:intro}Introduction}

There are a number of excellent molecular dynamics packages available
to the chemical physics
community.\cite{Brooks83,MacKerell98,pearlman:1995,Gromacs,Gromacs3,DL_POLY,Tinker,Paradyn,namd,macromodel}
All of these packages are stable, polished programs which solve many
problems of interest.  Most are now capable of performing molecular
dynamics simulations on parallel computers.  Some have source code
which is freely available to the entire scientific community.  Few,
however, are capable of efficiently integrating the equations of
motion for atom types with orientational degrees of freedom
(e.g. point dipoles, and ``sticky'' atoms).  And only one of the
programs referenced can handle transition metal force fields like the
Embedded Atom Method ({\sc eam}).  The direction our research program
has taken us now involves the use of atoms with orientational degrees
of freedom as well as transition metals.  Since these simulation
methods may be of some use to other researchers, we have decided to
release our program (and all related source code) to the scientific
community.

This paper communicates the algorithmic details of our program, which
we have been calling the Object-Oriented Parallel Simulation Engine
(i.e. {\sc oopse}).  We have structured this paper to first discuss
the underlying concepts in this simulation package
(Sec. \ref{oopseSec:IOfiles}).  The empirical energy functions
implemented are discussed in Sec.~\ref{oopseSec:empiricalEnergy}.
Sec.~\ref{oopseSec:mechanics} describes the various Molecular Dynamics
algorithms {\sc oopse} implements in the integration of Hamilton's
equations of motion.  Program design considerations for parallel
computing are presented in
Sec.~\ref{oopseSec:parallelization}. Concluding remarks are presented
in Sec.~\ref{oopseSec:conclusion}.

\section{\label{oopseSec:IOfiles}Concepts \& Files}

A simulation in {\sc oopse} is built using a few fundamental
conceptual building blocks most of which are chemically intuitive.
The basic unit of a simulation is an {\tt atom}.  The parameters
describing an {\tt atom} have been generalized to make it as flexible
as possible; this means that in addition to translational degrees of
freedom, {\tt Atoms} may also have {\it orientational} degrees of freedom.

The fundamental (static) properties of {\tt atoms} are defined by the
{\tt forceField} chosen for the simulation.  The atomic properties
specified by a {\tt forceField} might include (but are not limited to)
charge, $\sigma$ and $\epsilon$ values for Lennard-Jones interactions,
the strength of the dipole moment ($\mu$), the mass, and the moments
of inertia.  Other more complicated properties of atoms might also be
specified by the {\tt forceField}.

{\tt Atoms} can be grouped together in many ways.  A {\tt rigidBody}
contains atoms that exert no forces on one another and which move as a
single rigid unit.  A {\tt cutoffGroup} may contain atoms which
function together as a (rigid {\it or} non-rigid) unit for potential
energy calculations,
\begin{equation}
V_{ab} = s(r_{ab}) \sum_{i \in a} \sum_{j \in b} V_{ij}(r_{ij})
\end{equation}
Here, $a$ and $b$ are two {\tt cutoffGroups} containing multiple atoms
($a = \left\{i\right\}$ and $b = \left\{j\right\}$).  $s(r_{ab})$ is a
generalized switching function which insures that the atoms in the two
{\tt cutoffGroups} are treated identically as the two groups enter or
leave an interaction region.

{\tt Atoms} may also be grouped in more traditional ways into {\tt
bonds}, {\tt bends}, and {\tt torsions}.  These groupings allow the
correct choice of interaction parameters for short-range interactions
to be chosen from the definitions in the {\tt forceField}.

All of these groups of {\tt atoms} are brought together in the {\tt
molecule}, which is the fundamental structure for setting up and {\sc
oopse} simulation.  {\tt Molecules} contain lists of {\tt atoms}
followed by listings of the other atomic groupings ({\tt bonds}, {\tt
bends}, {\tt torsions}, {\tt rigidBodies}, and {\tt cutoffGroups})
which relate the atoms to one another.

Simulations often involve heterogeneous collections of molecules.  To
specify a mixture of {\tt molecule} types, {\sc oopse} uses {\tt
components}.  Even simulations containing only one type of molecule
must specify a single {\tt component}.

Starting a simulation requires two types of information: {\it
meta-data}, which describes the types of objects present in the
simulation, and {\it configuration} information, which describes the
initial state of these objects.  The meta-data is given to {\sc oopse}
using a C-based syntax that is parsed at the beginning of the
simulation.  Configuration information is specified using an extended
XYZ file format.  Both the meta-data and configuration file formats
are described in the following sections.

\subsection{Meta-data Files}

{\sc oopse} uses a C-based script syntax to parse the meta-data files
at run time.  These files allow the user to completely describe the
system they wish to simulate, as well as tailor {\sc oopse}'s behavior
during the simulation.  Meta-data files are typically denoted with the
extension {\tt .md} (which can stand for Meta-Data or Molecular
Dynamics or Molecule Definition depending on the user's mood). An
example meta-data file is shown in Scheme~\ref{sch:mdExample}.

\begin{lstlisting}[float,caption={[An example of a complete meta-data
file] An example showing a complete meta-data
file.},label={sch:mdExample}] 

molecule{
  name = "Ar";
  nAtoms = 1;
  atom[0]{
    type="Ar";
    position( 0.0, 0.0, 0.0 );
  }
}

nComponents = 1;
component{
  type = "Ar";
  nMol = 108;
}

initialConfig = "./argon.in";

forceField = "LJ";
ensemble = "NVE"; // specify the simulation ensemble
dt = 1.0;         // the time step for integration
runTime = 1e3;    // the total simulation run time
sampleTime = 100; // trajectory file frequency
statusTime = 50;  // statistics file frequency

\end{lstlisting}

Within the meta-data file it is necessary to provide a complete
description of the molecule before it is actually placed in the
simulation. {\sc oopse}'s meta-data syntax was originally developed
with this goal in mind, and allows for the use of {\it include files}
to specify all atoms in a molecular prototype, as well as any bonds,
bends, or torsions.  Include files allow the user to describe a
molecular prototype once, then simply include it into each simulation
containing that molecule. Returning to the example in
Scheme~\ref{sch:mdExample}, the include file's contents would be
Scheme~\ref{sch:mdIncludeExample}, and the new meta-data file would
become Scheme~\ref{sch:mdExPrime}.

\begin{lstlisting}[float,caption={An example molecule definition in an
include file.},label={sch:mdIncludeExample}]

molecule{
  name = "Ar";
  nAtoms = 1;
  atom[0]{
    type="Ar";
    position( 0.0, 0.0, 0.0 );
  }
}

\end{lstlisting}

\begin{lstlisting}[float,caption={Revised meta-data example.},label={sch:mdExPrime}]

#include "argon.md"

nComponents = 1;
component{
  type = "Ar";
  nMol = 108;
}

initialConfig = "./argon.in";

forceField = "LJ";
ensemble = "NVE";
dt = 1.0;
runTime = 1e3;
sampleTime = 100;
statusTime = 50; 

\end{lstlisting}

\subsection{\label{oopseSec:atomsMolecules}Atoms, Molecules, and other
ways of grouping atoms}

As mentioned above, the fundamental unit for an {\sc oopse} simulation
is the {\tt atom}.  Atoms can be collected into secondary structures
such as {\tt rigidBodies}, {\tt cutoffGroups}, or {\tt molecules}. The
{\tt molecule} is a way for {\sc oopse} to keep track of the atoms in
a simulation in logical manner. Molecular units store the identities
of all the atoms and rigid bodies associated with themselves, and they
are responsible for the evaluation of their own internal interactions
(\emph{i.e.}~bonds, bends, and torsions). Scheme
\ref{sch:mdIncludeExample} shows how one creates a molecule in an
included meta-data file. The positions of the atoms given in the
declaration are relative to the origin of the molecule, and the origin
is used when creating a system containing the molecule.

One of the features that sets {\sc oopse} apart from most of the
current molecular simulation packages is the ability to handle rigid
body dynamics. Rigid bodies are non-spherical particles or collections
of particles (e.g. $\mbox{C}_{60}$) that have a constant internal
potential and move collectively.\cite{Goldstein01} They are not
included in most simulation packages because of the algorithmic
complexity involved in propagating orientational degrees of freedom.
Integrators which propagate orientational motion with an acceptable
level of energy conservation for molecular dynamics are relatively
new inventions.  

Moving a rigid body involves determination of both the force and
torque applied by the surroundings, which directly affect the
translational and rotational motion in turn. In order to accumulate
the total force on a rigid body, the external forces and torques must
first be calculated for all the internal particles. The total force on
the rigid body is simply the sum of these external forces.
Accumulation of the total torque on the rigid body is more complex
than the force because the torque is applied to the center of mass of
the rigid body. The space-fixed torque on rigid body $i$ is
\begin{equation}
\boldsymbol{\tau}_i=
        \sum_{a}\biggl[(\mathbf{r}_{ia}-\mathbf{r}_i)\times \mathbf{f}_{ia} 
        + \boldsymbol{\tau}_{ia}\biggr],
\label{eq:torqueAccumulate}
\end{equation}
where $\boldsymbol{\tau}_i$ and $\mathbf{r}_i$ are the torque on and
position of the center of mass respectively, while $\mathbf{f}_{ia}$,
$\mathbf{r}_{ia}$, and $\boldsymbol{\tau}_{ia}$ are the force on,
position of, and torque on the component particles of the rigid body.

The summation of the total torque is done in the body fixed axis of
each rigid body. In order to move between the space fixed and body
fixed coordinate axes, parameters describing the orientation must be
maintained for each rigid body. At a minimum, the rotation matrix
($\mathsf{A}$) can be described by the three Euler angles ($\phi,
\theta,$ and $\psi$), where the elements of $\mathsf{A}$ are composed of
trigonometric operations involving $\phi, \theta,$ and
$\psi$.\cite{Goldstein01} In order to avoid numerical instabilities
inherent in using the Euler angles, the four parameter ``quaternion''
scheme is often used. The elements of $\mathsf{A}$ can be expressed as
arithmetic operations involving the four quaternions ($q_w, q_x, q_y,$
and $q_z$).\cite{allen87:csl} Use of quaternions also leads to
performance enhancements, particularly for very small
systems.\cite{Evans77}

Rather than use one of the previously stated methods, {\sc oopse}
utilizes a relatively new scheme that propagates the entire nine
parameter rotation matrix. Further discussion on this choice can be
found in Sec.~\ref{oopseSec:integrate}. An example definition of a
rigid body can be seen in Scheme
\ref{sch:rigidBody}.

\begin{lstlisting}[float,caption={[Defining rigid bodies]A sample
definition of a molecule containing a rigid body and a cutoff
group},label={sch:rigidBody}] 
molecule{
  name = "TIP3P";
  nAtoms = 3;
  atom[0]{
    type = "O_TIP3P";
    position( 0.0, 0.0, -0.06556 );
  }
  atom[1]{
    type = "H_TIP3P";
    position( 0.0, 0.75695, 0.52032 );
  }
  atom[2]{
    type = "H_TIP3P";
    position( 0.0, -0.75695, 0.52032 );
  }

  nRigidBodies = 1;
  rigidBody[0]{
    nMembers = 3;
    members(0, 1, 2);
  }

  nCutoffGroups = 1;
  cutoffGroup[0]{
    nMembers = 3;
    members(0, 1, 2);
  }
}
\end{lstlisting}

\subsection{\label{sec:miscConcepts}Creating a Metadata File}

The actual creation of a metadata file requires several key
components. The first part of the file needs to be the declaration of
all of the molecule prototypes used in the simulation. This is
typically done through included meta-data files. Only the molecules
actually present in the simulation need to be declared; however, {\sc
oopse} allows for the declaration of more molecules than are
needed. This gives the user the ability to build up a library of
commonly used molecules into a single include file.

Once all prototypes are declared, the ordering of the rest of the
script is less stringent.  The molecular composition of the simulation
is specified with {\tt component} statements. Each different type of
molecule present in the simulation is considered a separate
component. The number of components must be declared before the first
component block statement (an example is shown in
Sch.~\ref{sch:mdExPrime}).  The component blocks tell {\sc oopse} the
number of molecules that will be in the simulation, and the order in
which the components blocks are declared sets the ordering of the real
atoms in the configuration file as well as in the output files. The
remainder of the script then sets the various simulation parameters
for the system of interest.

The required set of parameters that must be present in all simulations
is given in Table~\ref{table:reqParams}.  Since the user can use {\sc
oopse} to perform energy minimizations as well as molecular dynamics
simulations, one of the {\tt minimizer} or {\tt ensemble} keywords
must be present.  The {\tt ensemble} keyword is responsible for
selecting the integration method used for the calculation of the
equations of motion. An in depth discussion of the various methods
available in {\sc oopse} can be found in
Sec.~\ref{oopseSec:mechanics}.  The {\tt minimizer} keyword selects
which minimization method to use, and more details on the choices of
minimizer parameters can be found in
Sec.~\ref{oopseSec:minimizer}. The {\tt forceField} statement is
important for the selection of which forces will be used in the course
of the simulation. {\sc oopse} supports several force fields, as
outlined in Sec.~\ref{oopseSec:empiricalEnergy}. The force fields are
interchangeable between simulations, with the only requirement being
that all atoms needed by the simulation are defined within the
selected force field. 

For molecular dynamics simulations, the time step between force
evaluations is set with the {\tt dt} parameter, and {\tt runTime} will
set the time length of the simulation. Note, that {\tt runTime} is an
absolute time, meaning if the simulation is started at t = 10.0~ns
with a {\tt runTime} of 25.0~ns, the simulation will only run for an
additional 15.0~ns.  

For energy minimizations, it is not necessary to specify {\tt dt} or
{\tt runTime}.

The final required parameter is the {\tt initialConfig}
statement. This will set the initial coordinates for the system, as
well as the initial time if the {\tt useInitalTime} flag is set to
{\tt true}. The format of the file specified in {\tt initialConfig},
is given in Sec.~\ref{oopseSec:coordFiles}. Additional parameters are
summarized in Table~\ref{table:genParams}.

It is important to note the fundamental units in all files which are
read and written by {\sc oopse}.  Energies are in $\mbox{kcal
mol}^{-1}$, distances are in $\mbox{\AA}$, times are in $\mbox{fs}$,
translational velocities are in $\mbox{\AA~fs}^{-1}$, and masses are
in $\mbox{amu}$.  Orientational degrees of freedom are described using
quaternions (unitless, but $q_w^2 + q_x^2 + q_y^2 + q_z^2 = 1$),
body-fixed angular momenta ($\mbox{amu \AA}^{2} \mbox{radians
fs}^{-1}$), and body-fixed moments of inertia ($\mbox{amu \AA}^{2}$). 

\begin{table}
\caption{Meta-data Keywords: Required Parameters}
\label{table:reqParams}
\begin{center}
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% of columns.
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{\bf keyword} & {\bf units} & {\bf use} & {\bf remarks} \\ \hline

{\tt forceField} & string & Sets the force field. & Possible force fields are DUFF, LJ, and EAM. \\
{\tt nComponents} & integer & Sets the number of components. & Needs to appear before the first {\tt Component} block. \\
{\tt initialConfig} & string & Sets the file containing the initial configuration. & Can point to any file containing the configuration in the correct order. \\
{\tt minimizer}& string & Chooses a minimizer & Possible minimizers
are SD and CG. Either {\tt ensemble} or {\tt minimizer} must be specified. \\
{\tt ensemble} & string & Sets the ensemble. & Possible ensembles are
NVE, NVT, NPTi, NPTf, and NPTxyz.  Either {\tt ensemble}
or {\tt minimizer} must be specified. \\
{\tt dt} & fs & Sets the time step. & Selection of {\tt dt} should be
small enough to sample the fastest motion of the simulation. ({\tt
dt} is required for molecular dynamics simulations)\\
{\tt runTime} & fs & Sets the time at which the simulation should
end. & This is an absolute time, and will end the simulation when the
current time meets or exceeds the {\tt runTime}. ({\tt runTime} is
required for molecular dynamics simulations)\\

\end{tabularx}
\end{center}
\end{table}

\begin{table}
\caption{Meta-data Keywords: General Parameters}
\label{table:genParams}
\begin{center}
% Note when adding or removing columns, the \hsize numbers must add up to the total number
% of columns.
\begin{tabularx}{\linewidth}%
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  >{\setlength{\hsize}{1.2\hsize}}X%
  >{\setlength{\hsize}{1.4\hsize}}X}

{\bf keyword} & {\bf units} & {\bf use} & {\bf remarks} \\ \hline

{\tt finalConfig} & string & Sets the name of the final
output file. & Useful when stringing simulations together. Defaults to
the root name of the initial meta-data file but with an {\tt .eor}
extension. \\ 
{\tt useInitialTime} & logical & Sets whether the initial time is taken from the {\tt .in} file. & Useful when recovering a simulation from a crashed processor. Default is false. \\
{\tt sampleTime} & fs & Sets the frequency at which the {\tt .dump}
file is written. & The default is equal to the {\tt runTime}. \\
{\tt statusTime} & fs & Sets the frequency at which the {\tt .stat}
file is written. & The default is equal to the {\tt sampleTime}. \\
{\tt cutoffRadius} & $\mbox{\AA}$ & Manually sets the cutoff radius &
Defaults to $15\mbox{\AA}$ for systems containing charges or dipoles or to $2.5
\sigma_{L}$, where $\sigma_{L}$ is the largest LJ $\sigma$ in the 
simulation. \\ 
{\tt switchingRadius} & $\mbox{\AA}$  & Manually sets the inner radius for the switching function. & Defaults to 95~\% of the {\tt cutoffRadius}. \\
{\tt useReactionField} & logical & Turns the reaction field correction on/off. & Default is false. \\
{\tt dielectric} & unitless & Sets the dielectric constant for reaction field. & If {\tt useReactionField} is true, then {\tt dielectric} must be set. \\
{\tt usePeriodicBoundaryConditions} & & & \\
        & logical & Turns periodic boundary conditions on/off. & Default is true. \\
{\tt seed } & integer & Sets the seed value for the random number
generator. & The seed needs to be at least 9 digits long. The default
is to take the seed from the CPU clock. \\
{\tt forceFieldVariant} & string & Sets the name of the variant of the
force field.  & {\sc eam} has three variants: {\tt u3}, {\tt u6}, and
{\tt VC}.

\end{tabularx}
\end{center}
\end{table}


\subsection{\label{oopseSec:coordFiles}Coordinate Files}

The standard format for storage of a systems coordinates is a modified
xyz-file syntax, the exact details of which can be seen in
Scheme~\ref{sch:dumpFormat}. As all bonding and molecular information
is stored in the meta-data files, the coordinate files contain only
the coordinates of the objects which move independently during the
simulation.  It is important to note that {\it not all atoms} are
capable of independent motion.  Atoms which are part of rigid bodies
are not ``integrable objects'' in the equations of motion; the rigid
bodies themselves are the integrable objects.  Therefore, the
coordinate file contains coordinates of all the {\tt
integrableObjects} in the system.  For systems without rigid bodies,
this is simply the coordinates of all the atoms. 

It is important to note that although the simulation propagates the
complete rotation matrix, directional entities are written out using
quaternions to save space in the output files.  All objects (atoms,
orientational atoms, and rigid bodies) are given quaternions and
angular momenta in coordinate files which are output by OOPSE, but it
is not necessary for the user to specify the quaternions or angular
momenta for atoms without orientational degrees of freedom.

\begin{lstlisting}[float,caption={[The format of the coordinate
files] An example of the format of the coordinate files. The fist line
is the number of {\tt integrableObjects} (freely-moving atoms and
rigid bodies). The second line begins with the time stamp followed by
the three $\mathsf{H}$ column vectors. It is important to note that
for extended system ensembles, additional information pertinent to the
integrators may be stored on this line as well. The next lines are the
coordinates for all integrable objects in the system.  The name of the
integrable object is followed by position, velocity, quaternions, and
lastly, body fixed angular momentum.},label=sch:dumpFormat]

nIntegrable
time; Hxx Hyx Hzx; Hxy Hyy Hzy; Hxz Hyz Hzz;
Name1 x y z vx vy vz qw qx qy qz jx jy jz
Name2 x y z vx vy vz qw qx qy qz jx jy jz
etc...

\end{lstlisting}

The {\tt name} field for atoms is simply the atom type as specified in
the meta-data file.  The {\tt name} field for a rigid body is
specified as {\tt MOLTYPE\_RB\_N}, to specify that this is {\tt
rigidBody} N in a molecule of type MOLTYPE.  In simulations with rigid
body models of water, a sample coordinate line might be:

\begin{tt}
TIP3P\_RB\_0  x y z vx vy vz qw qx qy qz jx jy jz
\end{tt}

which tells the program that the rigid body representing a TIP3P
molecule (rigid body \# 0) is listed on that line.

There are three files used by {\sc oopse} which are written in the
coordinate format.  They are: the initial coordinate file
(\texttt{.in}), the simulation trajectory file (\texttt{.dump}), and
the final coordinates or ``end-of-run'' for the simulation
(\texttt{.eor}). The initial coordinate file is necessary for {\sc
oopse} to start the simulation with the proper coordinates, and this
file must be generated by the user before the simulation run. The
trajectory (or ``dump'') file is updated during simulation and is used
to store snapshots of the coordinates at regular intervals. The first
frame is a duplication of the
\texttt{.in} file, and each subsequent frame is appended to the file
at an interval specified in the meta-data file with the
\texttt{sampleTime} flag. The final coordinate file is the
``end-of-run'' file.  The \texttt{.eor} file stores the final
configuration of the system for a given simulation. The file is
updated at the same time as the \texttt{.dump} file, but it only
contains the most recent frame. In this way, an \texttt{.eor} file may
be used to initialize a second simulation should it be necessary to
recover from a crash or power outage.

\subsection{\label{oopseSec:initCoords}Generation of Initial Coordinates}

As was stated in Sec.~\ref{oopseSec:coordFiles}, an initial coordinate
file is needed to provide the starting coordinates for a simulation.
Since each simulation is different, system creation is left to the end
user; however, we have included a few sample programs which make some
specialized structures.  The {\tt .in} file must list the integrable
objects in the correct order.  The ordering of the integrable objects
relies on the ordering of molecules within the meta-data file. {\sc
oopse} expects the order to comply with the following guidelines:
\begin{enumerate}
\item All of the molecules of the first declared component are given
before proceeding to the molecules of the second component, and so on
for all subsequently declared components.
\item The ordering of the atoms for each molecule follows the order
declared in the molecule's declaration within the model file.
\item Only atoms which are not members of a {\tt rigidBody} are
included.
\item Rigid Body coordinates for a molecule are listed immediately
after the the other atoms in a molecule.  Some molecules may be
entirely rigid, in which case, only the rigid body coordinates are
given.
\end{enumerate}
An example is given in the meta-data file in Scheme~\ref{sch:initEx1}
which results in the {\tt .in} file shown in Scheme~\ref{sch:initEx2}.

\begin{lstlisting}[float,caption={Example declaration of the
$\text{I}_2$ molecule and the HCl molecule. The two molecules are then
included into a simulation.}, label=sch:initEx1] 

molecule{
  name = "I2";
  nAtoms = 2;
  atom[0]{
    type = "I";
  }
  atom[1]{
    type = "I";
  }
  nBonds = 1;
  bond[0]{
    members( 0, 1);
  }
}

molecule{
  name = "HCl"
  nAtoms = 2;
  atom[0]{
    type = "H";
  }
  atom[1]{
    type = "Cl";
  }
  nBonds = 1;
  bond[0]{
    members( 0, 1);
  }
}

nComponents = 2;
component{
  type = "HCl";
  nMol = 4;
}
component{
  type = "I2";
  nMol = 1;
}

initialConfig = "mixture.in";

\end{lstlisting}

\begin{lstlisting}[float,caption={The contents of the {\tt
mixture.in} file matching the declarations in
Scheme~\ref{sch:initEx1}. Note that even though $\text{I}_2$ is
declared before HCl, the {\tt .in} file follows the order {\it in
which the components were included}.},label=sch:initEx2]

10
0.0;  10.0  0.0  0.0;  0.0  10.0  0.0;  0.0  0.0  10.0;
H  ...
Cl ...
H  ...
Cl ...
H  ...
Cl ...
H  ...
Cl ...
I  ...
I  ...

\end{lstlisting}


\subsection{The Statistics File}

The last output file generated by {\sc oopse} is the statistics
file. This file records such statistical quantities as the
instantaneous temperature (in $K$), volume (in $\mbox{\AA}^{3}$),
pressure (in $\mbox{atm}$), etc. It is written out with the frequency
specified in the meta-data file with the
\texttt{statusTime} keyword. The file allows the user to observe the
system variables as a function of simulation time while the simulation
is in progress. One useful function the statistics file serves is to
monitor the conserved quantity of a given simulation ensemble,
allowing the user to gauge the stability of the integrator. The
statistics file is denoted with the \texttt{.stat} file extension.

\section{\label{oopseSec:empiricalEnergy}The Empirical Energy
Functions}

Like many simulation packages, {\sc oopse} splits the potential energy
into the short-ranged (bonded) portion and a long-range (non-bonded)
potential,
\begin{equation}
V = V_{\mathrm{short-range}} + V_{\mathrm{long-range}}.
\end{equation}
The short-ranged portion includes the explicit bonds, bends, and
torsions which have been defined in the meta-data file for the
molecules which are present in the simulation.  The functional forms and
parameters for these interactions are defined by the force field which
is chosen.

Calculating the long-range (non-bonded) potential involves a sum over
all pairs of atoms (except for those atoms which are involved in a
bond, bend, or torsion with each other).  If done poorly, calculating
the the long-range interactions for $N$ atoms would involve $N(N-1)/2$
evaluations of atomic distances.  To reduce the number of distance
evaluations between pairs of atoms, {\sc oopse} uses a switched cutoff
with Verlet neighbor lists.\cite{allen87:csl} It is well known that
neutral groups which contain charges will exhibit pathological forces
unless the cutoff is applied to the neutral groups evenly instead of
to the individual atoms.\cite{leach01:mm} {\sc oopse} allows users to
specify cutoff groups which may contain an arbitrary number of atoms
in the molecule.  Atoms in a cutoff group are treated as a single unit
for the evaluation of the switching function:
\begin{equation}
V_{\mathrm{long-range}} = \sum_{a} \sum_{b>a} s(r_{ab}) \sum_{i \in a} \sum_{j \in b} V_{ij}(r_{ij}),
\end{equation}
where $r_{ab}$ is the distance between the centers of mass of the two
cutoff groups ($a$ and $b$).

The sums over $a$ and $b$ are over the cutoff groups that are present
in the simulation.  Atoms which are not explicitly defined as members
of a {\tt cutoffGroup} are treated as a group consisting of only one
atom.  The switching function, $s(r)$ is the standard cubic switching
function,
\begin{equation}
S(r) = 
        \begin{cases}
        1 & \text{if $r \le r_{\text{sw}}$},\\
        \frac{(r_{\text{cut}} + 2r - 3r_{\text{sw}})(r_{\text{cut}} - r)^2}
        {(r_{\text{cut}} - r_{\text{sw}})^2} 
        & \text{if $r_{\text{sw}} < r \le r_{\text{cut}}$}, \\
        0 & \text{if $r > r_{\text{cut}}$.}
        \end{cases}
\label{eq:dipoleSwitching}
\end{equation}
Here, $r_{\text{sw}}$ is the {\tt switchingRadius}, or the distance
beyond which interactions are reduced, and $r_{\text{cut}}$ is the
{\tt cutoffRadius}, or the distance at which interactions are
truncated.

Users of {\sc oopse} do not need to specify the {\tt cutoffRadius} or
{\tt switchingRadius}.  In simulations containing only Lennard-Jones
atoms, the cutoff radius has a default value of $2.5\sigma_{ii}$,
where $\sigma_{ii}$ is the largest Lennard-Jones length parameter
present in the simulation.  In simulations containing charged or
dipolar atoms, the default cutoff radius is $15 \mbox{\AA}$.  

The {\tt switchingRadius} is set to a default value of 95\% of the
{\tt cutoffRadius}.  In the special case of a simulation containing
{\it only} Lennard-Jones atoms, the default switching radius takes the
same value as the cutoff radius, and {\sc oopse} will use a shifted
potential to remove discontinuities in the potential at the cutoff.
Both radii may be specified in the meta-data file.

Force fields can be added to {\sc oopse}, although it comes with a few
simple examples (Lennard-Jones, {\sc duff}, {\sc water}, and {\sc
eam}) which are explained in the following sections.

\subsection{\label{sec:LJPot}The Lennard Jones Force Field}

The most basic force field implemented in {\sc oopse} is the
Lennard-Jones force field, which mimics the van der Waals interaction
at long distances and uses an empirical repulsion at short
distances. The Lennard-Jones potential is given by:
\begin{equation}
V_{\text{LJ}}(r_{ij}) = 
        4\epsilon_{ij} \biggl[
        \biggl(\frac{\sigma_{ij}}{r_{ij}}\biggr)^{12}
        - \biggl(\frac{\sigma_{ij}}{r_{ij}}\biggr)^{6}
        \biggr],
\label{eq:lennardJonesPot}
\end{equation}
where $r_{ij}$ is the distance between particles $i$ and $j$,
$\sigma_{ij}$ scales the length of the interaction, and
$\epsilon_{ij}$ scales the well depth of the potential. Scheme
\ref{sch:LJFF} gives an example meta-data file that
sets up a system of 108 Ar particles to be simulated using the
Lennard-Jones force field.

\begin{lstlisting}[float,caption={[Invocation of the Lennard-Jones
force field] A sample meta-data file for a small Lennard-Jones
simulation.},label={sch:LJFF}]

#include "argon.md" 

nComponents = 1;
component{
  type = "Ar";
  nMol = 108;
}

initialConfig = "./argon.in";

forceField = "LJ";
\end{lstlisting}

Interactions between dissimilar particles requires the generation of
cross term parameters for $\sigma$ and $\epsilon$. These parameters
are determined using the Lorentz-Berthelot mixing
rules:\cite{allen87:csl}
\begin{equation}
\sigma_{ij} = \frac{1}{2}[\sigma_{ii} + \sigma_{jj}],
\label{eq:sigmaMix}
\end{equation}
and
\begin{equation}
\epsilon_{ij} = \sqrt{\epsilon_{ii} \epsilon_{jj}}.
\label{eq:epsilonMix}
\end{equation}

\subsection{\label{oopseSec:DUFF}Dipolar Unified-Atom Force Field}

The dipolar unified-atom force field ({\sc duff}) was developed to
simulate lipid bilayers. These types of simulations require a model
capable of forming bilayers, while still being sufficiently
computationally efficient to allow large systems ($\sim$100's of
phospholipids, $\sim$1000's of waters) to be simulated for long times
($\sim$10's of nanoseconds). With this goal in mind, {\sc duff} has no
point charges. Charge-neutral distributions are replaced with dipoles,
while most atoms and groups of atoms are reduced to Lennard-Jones
interaction sites. This simplification reduces the length scale of
long range interactions from $\frac{1}{r}$ to $\frac{1}{r^3}$,
removing the need for the computationally expensive Ewald
sum. Instead, Verlet neighbor-lists and cutoff radii are used for the
dipolar interactions, and, if desired, a reaction field may be added
to mimic longer range interactions.

As an example, lipid head-groups in {\sc duff} are represented as
point dipole interaction sites.  Placing a dipole at the head group's
center of mass mimics the charge separation found in common
phospholipid head groups such as phosphatidylcholine.\cite{Cevc87}
Additionally, a large Lennard-Jones site is located at the
pseudoatom's center of mass. The model is illustrated by the red atom
in Fig.~\ref{oopseFig:lipidModel}. The water model we use to
complement the dipoles of the lipids is a
reparameterization\cite{fennell04} of the soft sticky dipole (SSD)
model of Ichiye
\emph{et al.}\cite{liu96:new_model}

\begin{figure}
\centering
\includegraphics[width=\linewidth]{lipidModel.eps}
\caption[A representation of a lipid model in {\sc duff}]{A
representation of the lipid model. $\phi$ is the torsion angle,
$\theta$ is the bend angle, and $\mu$ is the dipole moment of the head
group.}
\label{oopseFig:lipidModel}
\end{figure}

A set of scalable parameters has been used to model the alkyl groups
with Lennard-Jones sites. For this, parameters from the TraPPE force
field of Siepmann \emph{et al.}\cite{Siepmann1998} have been
utilized. TraPPE is a unified-atom representation of n-alkanes which
is parametrized against phase equilibria using Gibbs ensemble Monte
Carlo simulation techniques.\cite{Siepmann1998} One of the advantages
of TraPPE is that it generalizes the types of atoms in an alkyl chain
to keep the number of pseudoatoms to a minimum; thus, the parameters
for a unified atom such as $\text{CH}_2$ do not change depending on
what species are bonded to it.

As is required by TraPPE, {\sc duff} also constrains all bonds to be
of fixed length. Typically, bond vibrations are the fastest motions in
a molecular dynamic simulation.  With these vibrations present, small
time steps between force evaluations must be used to ensure adequate
energy conservation in the bond degrees of freedom. By constraining
the bond lengths, larger time steps may be used when integrating the
equations of motion. A simulation using {\sc duff} is illustrated in
Scheme \ref{sch:DUFF}.

\begin{lstlisting}[float,caption={[Invocation of {\sc duff}]A portion
of a meta-data file showing a simulation utilizing {\sc
duff}},label={sch:DUFF}]  

#include "water.md"
#include "lipid.md"

nComponents = 2;
component{
  type = "simpleLipid_16";
  nMol = 60;
}

component{
  type = "SSD_water";
  nMol = 1936;
}

initialConfig = "bilayer.in";

forceField = "DUFF";

\end{lstlisting}

\subsubsection{\label{oopseSec:energyFunctions}{\sc duff} Energy Functions}

The total potential energy function in {\sc duff} is
\begin{equation}
V = \sum^{N}_{I=1} V^{I}_{\text{Internal}}
        + \sum^{N-1}_{I=1} \sum_{J>I} V^{IJ}_{\text{Cross}},
\label{eq:totalPotential}
\end{equation}
where $V^{I}_{\text{Internal}}$ is the internal potential of molecule $I$:
\begin{equation}
 V^{I}_{\text{Internal}} = 
        \sum_{\theta_{ijk} \in I} V_{\text{bend}}(\theta_{ijk})
        + \sum_{\phi_{ijkl} \in I} V_{\text{torsion}}(\phi_{ijkl})
        + \sum_{i \in I} \sum_{(j>i+4) \in I} 
        \biggl[ V_{\text{LJ}}(r_{ij}) +  V_{\text{dipole}}
        (\mathbf{r}_{ij},\boldsymbol{\Omega}_{i},\boldsymbol{\Omega}_{j})
        \biggr].
\label{eq:internalPotential}
\end{equation}
Here $V_{\text{bend}}$ is the bend potential for all 1, 3 bonded pairs
within the molecule $I$, and $V_{\text{torsion}}$ is the torsion
potential for all 1, 4 bonded pairs.  The pairwise portions of the
non-bonded interactions are excluded for atom pairs that are involved
in the smae bond, bend, or torsion. All other atom pairs within a
molecule are subject to the LJ pair potential.

The bend potential of a molecule is represented by the following function:
\begin{equation}
V_{\text{bend}}(\theta_{ijk}) = k_{\theta}( \theta_{ijk} - \theta_0
)^2, \label{eq:bendPot}
\end{equation}
where $\theta_{ijk}$ is the angle defined by atoms $i$, $j$, and $k$
(see Fig.~\ref{oopseFig:lipidModel}), $\theta_0$ is the equilibrium
bond angle, and $k_{\theta}$ is the force constant which determines the
strength of the harmonic bend. The parameters for $k_{\theta}$ and
$\theta_0$ are borrowed from those in TraPPE.\cite{Siepmann1998}

The torsion potential and parameters are also borrowed from TraPPE. It is
of the form:
\begin{equation}
V_{\text{torsion}}(\phi) = c_1[1 + \cos \phi] 
        + c_2[1 + \cos(2\phi)] 
        + c_3[1 + \cos(3\phi)],
\label{eq:origTorsionPot}
\end{equation}
where:
\begin{equation}
\cos\phi = (\hat{\mathbf{r}}_{ij} \times \hat{\mathbf{r}}_{jk}) \cdot
        (\hat{\mathbf{r}}_{jk} \times \hat{\mathbf{r}}_{kl}).
\label{eq:torsPhi}
\end{equation}
Here, $\hat{\mathbf{r}}_{\alpha\beta}$ are the set of unit bond
vectors between atoms $i$, $j$, $k$, and $l$. For computational
efficiency, the torsion potential has been recast after the method of
{\sc charmm},\cite{Brooks83} in which the angle series is converted to
a power series of the form:
\begin{equation}
V_{\text{torsion}}(\phi) =  
        k_3 \cos^3 \phi + k_2 \cos^2 \phi + k_1 \cos \phi + k_0,
\label{eq:torsionPot}
\end{equation}
where:
\begin{align*}
k_0 &= c_1 + c_3, \\
k_1 &= c_1 - 3c_3, \\
k_2 &= 2 c_2, \\
k_3 &= 4c_3.
\end{align*}
By recasting the potential as a power series, repeated trigonometric
evaluations are avoided during the calculation of the potential
energy.


The cross potential between molecules $I$ and $J$,
$V^{IJ}_{\text{Cross}}$, is as follows:
\begin{equation}
V^{IJ}_{\text{Cross}} = 
        \sum_{i \in I} \sum_{j \in J}
        \biggl[ V_{\text{LJ}}(r_{ij}) +  V_{\text{dipole}}
        (\mathbf{r}_{ij},\boldsymbol{\Omega}_{i},\boldsymbol{\Omega}_{j})
        + V_{\text{sticky}}
        (\mathbf{r}_{ij},\boldsymbol{\Omega}_{i},\boldsymbol{\Omega}_{j})
        \biggr],
\label{eq:crossPotentail}
\end{equation}
where $V_{\text{LJ}}$ is the Lennard Jones potential,
$V_{\text{dipole}}$ is the dipole dipole potential, and
$V_{\text{sticky}}$ is the sticky potential defined by the SSD model
(Sec.~\ref{oopseSec:SSD}). Note that not all atom types include all
interactions.

The dipole-dipole potential has the following form:
\begin{equation}
V_{\text{dipole}}(\mathbf{r}_{ij},\boldsymbol{\Omega}_{i},
        \boldsymbol{\Omega}_{j}) = \frac{|\mu_i||\mu_j|}{4\pi\epsilon_{0}r_{ij}^{3}} \biggl[
        \boldsymbol{\hat{u}}_{i} \cdot \boldsymbol{\hat{u}}_{j}
        -
        3(\boldsymbol{\hat{u}}_i \cdot \hat{\mathbf{r}}_{ij}) %
                (\boldsymbol{\hat{u}}_j \cdot \hat{\mathbf{r}}_{ij}) \biggr].
\label{eq:dipolePot}
\end{equation}
Here $\mathbf{r}_{ij}$ is the vector starting at atom $i$ pointing
towards $j$, and $\boldsymbol{\Omega}_i$ and $\boldsymbol{\Omega}_j$
are the orientational degrees of freedom for atoms $i$ and $j$
respectively. The magnitude of the dipole moment of atom $i$ is
$|\mu_i|$, $\boldsymbol{\hat{u}}_i$ is the standard unit orientation
vector of $\boldsymbol{\Omega}_i$, and $\boldsymbol{\hat{r}}_{ij}$ is
the unit vector pointing along $\mathbf{r}_{ij}$
($\boldsymbol{\hat{r}}_{ij}=\mathbf{r}_{ij}/|\mathbf{r}_{ij}|$).

\subsubsection{\label{oopseSec:SSD}The {\sc duff} Water Models: SSD/E
and SSD/RF} 

In the interest of computational efficiency, the default solvent used
by {\sc oopse} is the extended Soft Sticky Dipole (SSD/E) water
model.\cite{fennell04} The original SSD was developed by Ichiye
\emph{et al.}\cite{liu96:new_model} as a modified form of the hard-sphere 
water model proposed by Bratko, Blum, and
Luzar.\cite{Bratko85,Bratko95} It consists of a single point dipole
with a Lennard-Jones core and a sticky potential that directs the
particles to assume the proper hydrogen bond orientation in the first
solvation shell. Thus, the interaction between two SSD water molecules
\emph{i} and \emph{j} is given by the potential
\begin{equation}
V_{ij} = 
        V_{ij}^{LJ} (r_{ij})\ + V_{ij}^{dp}
        (\mathbf{r}_{ij},\boldsymbol{\Omega}_i,\boldsymbol{\Omega}_j)\ +
        V_{ij}^{sp}
        (\mathbf{r}_{ij},\boldsymbol{\Omega}_i,\boldsymbol{\Omega}_j),
\label{eq:ssdPot}
\end{equation}
where the $\mathbf{r}_{ij}$ is the position vector between molecules
\emph{i} and \emph{j} with magnitude equal to the distance $r_{ij}$, and
$\boldsymbol{\Omega}_i$ and $\boldsymbol{\Omega}_j$ represent the
orientations of the respective molecules. The Lennard-Jones and dipole
parts of the potential are given by equations \ref{eq:lennardJonesPot}
and \ref{eq:dipolePot} respectively. The sticky part is described by
the following,
\begin{equation}
u_{ij}^{sp}(\mathbf{r}_{ij},\boldsymbol{\Omega}_i,\boldsymbol{\Omega}_j)=
        \frac{\nu_0}{2}[s(r_{ij})w(\mathbf{r}_{ij},
        \boldsymbol{\Omega}_i,\boldsymbol{\Omega}_j) +
        s^\prime(r_{ij})w^\prime(\mathbf{r}_{ij},
        \boldsymbol{\Omega}_i,\boldsymbol{\Omega}_j)]\ ,
\label{eq:stickyPot}
\end{equation}
where $\nu_0$ is a strength parameter for the sticky potential, and
$s$ and $s^\prime$ are cubic switching functions which turn off the
sticky interaction beyond the first solvation shell. The $w$ function
can be thought of as an attractive potential with tetrahedral
geometry:
\begin{equation}
w({\bf r}_{ij},{\bf \Omega}_i,{\bf \Omega}_j)=
        \sin\theta_{ij}\sin2\theta_{ij}\cos2\phi_{ij},
\label{eq:stickyW}
\end{equation}
while the $w^\prime$ function counters the normal aligned and
anti-aligned structures favored by point dipoles:
\begin{equation}
w^\prime({\bf r}_{ij},{\bf \Omega}_i,{\bf \Omega}_j)=
        (\cos\theta_{ij}-0.6)^2(\cos\theta_{ij}+0.8)^2-w^0,
\label{eq:stickyWprime}
\end{equation}
It should be noted that $w$ is proportional to the sum of the $Y_3^2$
and $Y_3^{-2}$ spherical harmonics (a linear combination which
enhances the tetrahedral geometry for hydrogen bonded structures),
while $w^\prime$ is a purely empirical function.  A more detailed
description of the functional parts and variables in this potential
can be found in the original SSD
articles.\cite{liu96:new_model,liu96:monte_carlo,chandra99:ssd_md,Ichiye03}

\begin{figure}
\centering
\includegraphics[width=\linewidth]{waterAngle.eps}
\caption[Coordinate definition for the SSD/E water model]{Coordinates
for the interaction between two SSD/E water molecules.  $\theta_{ij}$
is the angle that $r_{ij}$ makes with the $\hat{z}$ vector in the
body-fixed frame for molecule $i$.  The $\hat{z}$ vector bisects the
HOH angle in each water molecule. } 
\label{oopseFig:ssd}
\end{figure}


Since SSD/E is a single-point {\it dipolar} model, the force
calculations are simplified significantly relative to the standard
{\it charged} multi-point models. In the original Monte Carlo
simulations using this model, Ichiye {\it et al.} reported that using
SSD decreased computer time by a factor of 6-7 compared to other
models.\cite{liu96:new_model} What is most impressive is that these
savings did not come at the expense of accurate depiction of the
liquid state properties.  Indeed, SSD/E maintains reasonable agreement
with the Head-Gordon diffraction data for the structural features of
liquid water.\cite{hura00,liu96:new_model} Additionally, the dynamical
properties exhibited by SSD/E agree with experiment better than those
of more computationally expensive models (like TIP3P and
SPC/E).\cite{chandra99:ssd_md} The combination of speed and accurate
depiction of solvent properties makes SSD/E a very attractive model
for the simulation of large scale biochemical simulations.

Recent constant pressure simulations revealed issues in the original
SSD model that led to lower than expected densities at all target
pressures.\cite{Ichiye03,fennell04} The default model in {\sc oopse}
is therefore SSD/E, a density corrected derivative of SSD that
exhibits improved liquid structure and transport behavior. If the use
of a reaction field long-range interaction correction is desired, it
is recommended that the parameters be modified to those of the SSD/RF
model (an SSD variant parameterized for reaction field). These solvent
parameters are listed and can be easily modified in the {\sc duff}
force field file ({\tt DUFF.frc}).  A table of the parameter values
and the drawbacks and benefits of the different density corrected SSD
models can be found in reference~\citen{fennell04}. 

\subsection{\label{oopseSec:WATER}The {\sc water} Force Field}

In addition to the {\sc duff} force field's solvent description, a
separate {\sc water} force field has been included for simulating most
of the common rigid-body water models. This force field includes the
simple and point-dipolar models (SSD, SSD1, SSD/E, SSD/RF, and DPD
water), as well as the common charge-based models (SPC, SPC/E, TIP3P,
TIP4P, and
TIP5P).\cite{liu96:new_model,Ichiye03,fennell04,Marrink01,Berendsen81,Berendsen87,Jorgensen83,Mahoney00}
In order to handle these models, charge-charge interactions were
included in the force-loop:
\begin{equation}
V_{\text{charge}}(r_{ij}) = \sum_{ij}\frac{q_iq_je^2}{r_{ij}}, 
\end{equation}
where $q$ represents the charge on particle $i$ or $j$, and $e$ is the
charge of an electron in Coulombs.  As with the other pair
interactions, charges can be simulated with a pure cutoff or a
reaction field.  The {\sc water} force field can be easily expanded
through modification of the {\sc water} force field file ({\tt
WATER.frc}). By adding atom types and inserting the appropriate
parameters, it is possible to extend the force field to handle rigid
molecules other than water.

\subsection{\label{oopseSec:eam}Embedded Atom Method}

{\sc oopse} implements a potential that describes bonding in
transition metal
systems.~\cite{Finnis84,Ercolessi88,Chen90,Qi99,Ercolessi02} This
potential has an attractive interaction which models ``Embedding'' a
positively charged pseudo-atom core in the electron density due to the
free valance ``sea'' of electrons created by the surrounding atoms in
the system.  A pairwise part of the potential (which is primarily
repulsive) describes the interaction of the positively charged metal
core ions with one another.  The Embedded Atom Method ({\sc
eam})~\cite{Daw84,FBD86,johnson89,Lu97} has been widely adopted in the
materials science community and has been included in {\sc oopse}. A
good review of {\sc eam} and other formulations of metallic potentials
was given by Voter.\cite{Voter:95}

The {\sc eam} potential has the form:
\begin{equation}
V  =  \sum_{i} F_{i}\left[\rho_{i}\right] + \sum_{i} \sum_{j \neq i}
\phi_{ij}({\bf r}_{ij})
\end{equation}
where $F_{i} $ is an embedding functional that approximates the energy
required to embed a positively-charged core ion $i$ into a linear
superposition of spherically averaged atomic electron densities given
by $\rho_{i}$,
\begin{equation}
\rho_{i}   =  \sum_{j \neq i} f_{j}({\bf r}_{ij}),
\end{equation}
Since the density at site $i$ ($\rho_i$) must be computed before the
embedding functional can be evaluated, {\sc eam} and the related
transition metal potentials require two loops through the atom pairs
to compute the inter-atomic forces.

The pairwise portion of the potential, $\phi_{ij}$, is a primarily
repulsive interaction between atoms $i$ and $j$. In the original
formulation of {\sc eam}\cite{Daw84}, $\phi_{ij}$ was an entirely
repulsive term; however later refinements to {\sc eam} allowed for
more general forms for $\phi$.\cite{Daw89} The effective cutoff
distance, $r_{{\text cut}}$ is the distance at which the values of
$f(r)$ and $\phi(r)$ drop to zero for all atoms present in the
simulation.  In practice, this distance is fairly small, limiting the
summations in the {\sc eam} equation to the few dozen atoms
surrounding atom $i$ for both the density $\rho$ and pairwise $\phi$
interactions.

In computing forces for alloys, mixing rules as outlined by
Johnson~\cite{johnson89} are used to compute the heterogenous pair
potential,
\begin{eqnarray}
\label{eq:johnson}
\phi_{ab}(r)=\frac{1}{2}\left(
\frac{f_{b}(r)}{f_{a}(r)}\phi_{aa}(r)+
\frac{f_{a}(r)}{f_{b}(r)}\phi_{bb}(r)
\right).
\end{eqnarray}
No mixing rule is needed for the densities, since the density at site
$i$ is simply the linear sum of density contributions of all the other
atoms.

The {\sc eam} force field illustrates an additional feature of {\sc
oopse}.  Foiles, Baskes and Daw fit {\sc eam} potentials for Cu, Ag,
Au, Ni, Pd, Pt and alloys of these metals.\cite{FBD86} These fits are
included in {\sc oopse} as the {\tt u3} variant of the {\sc eam} force
field.  Voter and Chen reparamaterized a set of {\sc eam} functions
which do a better job of predicting melting points.\cite{Voter:87}
These functions are included in {\sc oopse} as the {\tt VC} variant of
the {\sc eam} force field.  An additional set of functions (the
``Universal 6'' functions) are included in {\sc oopse} as the {\tt u6}
variant of {\sc eam}.  For example, to specify the Voter-Chen variant
of the {\sc eam} force field, the user would add the {\tt
forceFieldVariant = "VC";} line to the meta-data file.

The potential files used by the {\sc eam} force field are in the
standard {\tt funcfl} format, which is the format utilized by a number
of other codes (e.g. ParaDyn~\cite{Paradyn}, {\sc dynamo 86}).  It
should be noted that the energy units in these files are in eV, not
$\mbox{kcal mol}^{-1}$ as in the rest of the {\sc oopse} force field
files.  

\subsection{\label{oopseSec:pbc}Periodic Boundary Conditions} 

\newcommand{\roundme}{\operatorname{round}}

\textit{Periodic boundary conditions} are widely used to simulate bulk
properties with a relatively small number of particles. In this method
the simulation box is replicated throughout space to form an infinite
lattice.  During the simulation, when a particle moves in the primary
cell, its image in other cells move in exactly the same direction with
exactly the same orientation. Thus, as a particle leaves the primary
cell, one of its images will enter through the opposite face. If the
simulation box is large enough to avoid ``feeling'' the symmetries of
the periodic lattice, surface effects can be ignored. The available
periodic cells in {\sc oopse} are cubic, orthorhombic and
parallelepiped.  {\sc oopse} use a $3 \times 3$ matrix, $\mathsf{H}$,
to describe the shape and size of the simulation box. $\mathsf{H}$ is
defined:
\begin{equation}
\mathsf{H} = ( \mathbf{h}_x, \mathbf{h}_y, \mathbf{h}_z ),
\end{equation}
where $\mathbf{h}_{\alpha}$ is the column vector of the $\alpha$ axis of the
box.  During the course of the simulation both the size and shape of
the box can be changed to allow volume fluctuations when constraining
the pressure.

A real space vector, $\mathbf{r}$ can be transformed in to a box space
vector, $\mathbf{s}$, and back through the following transformations:
\begin{align}
\mathbf{s} &= \mathsf{H}^{-1} \mathbf{r}, \\
\mathbf{r} &= \mathsf{H} \mathbf{s}.
\end{align}
The vector $\mathbf{s}$ is now a vector expressed as the number of box
lengths in the $\mathbf{h}_x$, $\mathbf{h}_y$, and $\mathbf{h}_z$
directions. To find the minimum image of a vector $\mathbf{r}$, {\sc
oopse} first converts it to its corresponding vector in box space, and
then casts each element to lie in the range $[-0.5,0.5]$:
\begin{equation}
s_{i}^{\prime}=s_{i}-\roundme(s_{i}),
\end{equation}
where $s_i$ is the $i$th element of $\mathbf{s}$, and
$\roundme(s_i)$ is given by
\begin{equation}
\roundme(x) =
        \begin{cases}
        \lfloor x+0.5 \rfloor & \text{if $x \ge 0$,} \\
        \lceil x-0.5 \rceil & \text{if $x < 0$.}
        \end{cases}
\end{equation}
Here $\lfloor x \rfloor$ is the floor operator, and gives the largest
integer value that is not greater than $x$, and $\lceil x \rceil$ is
the ceiling operator, and gives the smallest integer that is not less
than $x$. 

Finally, the minimum image coordinates $\mathbf{r}^{\prime}$ are
obtained by transforming back to real space,
\begin{equation}
\mathbf{r}^{\prime}=\mathsf{H}^{-1}\mathbf{s}^{\prime}.%
\end{equation}
In this way, particles are allowed to diffuse freely in $\mathbf{r}$,
but their minimum images, or $\mathbf{r}^{\prime}$, are used to compute
the inter-atomic forces.


\section{\label{oopseSec:mechanics}Mechanics}

\subsection{\label{oopseSec:integrate}Integrating the Equations of Motion: the
{\sc dlm} method}

The default method for integrating the equations of motion in {\sc
oopse} is a velocity-Verlet version of the symplectic splitting method
proposed by Dullweber, Leimkuhler and McLachlan
({\sc dlm}).\cite{Dullweber1997} When there are no directional atoms or
rigid bodies present in the simulation, this integrator becomes the
standard velocity-Verlet integrator which is known to sample the
microcanonical (NVE) ensemble.\cite{Frenkel1996}

Previous integration methods for orientational motion have problems
that are avoided in the {\sc dlm} method.  Direct propagation of the Euler
angles has a known $1/\sin\theta$ divergence in the equations of
motion for $\phi$ and $\psi$,\cite{allen87:csl} leading to numerical
instabilities any time one of the directional atoms or rigid bodies
has an orientation near $\theta=0$ or $\theta=\pi$.  Quaternion-based
integration methods work well for propagating orientational motion;
however, energy conservation concerns arise when using the
microcanonical (NVE) ensemble.  An earlier implementation of {\sc
oopse} utilized quaternions for propagation of rotational motion;
however, a detailed investigation showed that they resulted in a
steady drift in the total energy, something that has been observed by
Laird {\it et al.}\cite{Laird97}

The key difference in the integration method proposed by Dullweber
\emph{et al.} is that the entire $3 \times 3$ rotation matrix is
propagated from one time step to the next. In the past, this would not
have been feasible, since the rotation matrix for a single body has
nine elements compared with the more memory-efficient methods (using
three Euler angles or 4 quaternions).  Computer memory has become much
less costly in recent years, and this can be translated into
substantial benefits in energy conservation.

The basic equations of motion being integrated are derived from the
Hamiltonian for conservative systems containing rigid bodies,
\begin{equation}
H = \sum_{i} \left( \frac{1}{2} m_i {\bf v}_i^T \cdot {\bf v}_i +
\frac{1}{2} {\bf j}_i^T \cdot \overleftrightarrow{\mathsf{I}}_i^{-1} \cdot
{\bf j}_i \right) +
V\left(\left\{{\bf r}\right\}, \left\{\mathsf{A}\right\}\right),
\end{equation}
where ${\bf r}_i$ and ${\bf v}_i$ are the cartesian position vector
and velocity of the center of mass of particle $i$, and ${\bf j}_i$,
$\overleftrightarrow{\mathsf{I}}_i$ are the body-fixed angular
momentum and moment of inertia tensor respectively, and the
superscript $T$ denotes the transpose of the vector.  $\mathsf{A}_i$
is the $3 \times 3$ rotation matrix describing the instantaneous
orientation of the particle.  $V$ is the potential energy function
which may depend on both the positions $\left\{{\bf r}\right\}$ and
orientations $\left\{\mathsf{A}\right\}$ of all particles.  The
equations of motion for the particle centers of mass are derived from
Hamilton's equations and are quite simple,
\begin{eqnarray}
\dot{{\bf r}} & = & {\bf v}, \\
\dot{{\bf v}} & = & \frac{{\bf f}}{m},
\end{eqnarray}
where ${\bf f}$ is the instantaneous force on the center of mass
of the particle,
\begin{equation}
{\bf f} = - \frac{\partial}{\partial
{\bf r}} V(\left\{{\bf r}(t)\right\}, \left\{\mathsf{A}(t)\right\}).
\end{equation}

The equations of motion for the orientational degrees of freedom are
\begin{eqnarray}
\dot{\mathsf{A}} & = & \mathsf{A} \cdot
\mbox{ skew}\left(\overleftrightarrow{\mathsf{I}}^{-1} \cdot {\bf j}\right),\\
\dot{{\bf j}} & = & {\bf j} \times \left( \overleftrightarrow{\mathsf{I}}^{-1}
\cdot {\bf j} \right) - \mbox{ rot}\left(\mathsf{A}^{T} \cdot \frac{\partial
V}{\partial \mathsf{A}} \right).
\end{eqnarray}
In these equations of motion, the $\mbox{skew}$ matrix of a vector
${\bf v} = \left( v_1, v_2, v_3 \right)$ is defined:
\begin{equation}
\mbox{skew}\left( {\bf v} \right) := \left( 
\begin{array}{ccc}
0 & v_3 & - v_2 \\
-v_3 & 0 & v_1 \\
v_2 & -v_1 & 0 
\end{array}
\right).
\end{equation}
The $\mbox{rot}$ notation refers to the mapping of the $3 \times 3$
rotation matrix to a vector of orientations by first computing the
skew-symmetric part $\left(\mathsf{A} - \mathsf{A}^{T}\right)$ and
then associating this with a length 3 vector by inverting the
$\mbox{skew}$ function above:
\begin{equation}
\mbox{rot}\left(\mathsf{A}\right) := \mbox{ skew}^{-1}\left(\mathsf{A}
- \mathsf{A}^{T} \right).
\end{equation}
Written this way, the $\mbox{rot}$ operation creates a set of
conjugate angle coordinates to the body-fixed angular momenta
represented by ${\bf j}$.  This equation of motion for angular momenta
is equivalent to the more familiar body-fixed forms,
\begin{eqnarray}
\dot{j_{x}} & = & \tau^b_x(t)  -
\left(\overleftrightarrow{\mathsf{I}}_{yy}^{-1} - \overleftrightarrow{\mathsf{I}}_{zz}^{-1} \right) j_y j_z, \\
\dot{j_{y}} & = & \tau^b_y(t) -
\left(\overleftrightarrow{\mathsf{I}}_{zz}^{-1} - \overleftrightarrow{\mathsf{I}}_{xx}^{-1} \right) j_z j_x,\\
\dot{j_{z}} & = & \tau^b_z(t) -
\left(\overleftrightarrow{\mathsf{I}}_{xx}^{-1} - \overleftrightarrow{\mathsf{I}}_{yy}^{-1} \right) j_x j_y, 
\end{eqnarray}
which utilize the body-fixed torques, ${\bf \tau}^b$. Torques are
most easily derived in the space-fixed frame, 
\begin{equation}
{\bf \tau}^b(t) = \mathsf{A}(t) \cdot {\bf \tau}^s(t),
\end{equation}
where the torques are either derived from the forces on the
constituent atoms of the rigid body, or for directional atoms,
directly from derivatives of the potential energy,
\begin{equation}
{\bf \tau}^s(t) = - \hat{\bf u}(t) \times \left( \frac{\partial}
{\partial \hat{\bf u}} V\left(\left\{ {\bf r}(t) \right\}, \left\{
\mathsf{A}(t) \right\}\right) \right).
\end{equation}
Here $\hat{\bf u}$ is a unit vector pointing along the principal axis
of the particle in the space-fixed frame.

The {\sc dlm} method uses a Trotter factorization of the orientational
propagator.  This has three effects:
\begin{enumerate}
\item the integrator is area-preserving in phase space (i.e. it is
{\it symplectic}),
\item the integrator is time-{\it reversible}, making it suitable for Hybrid
Monte Carlo applications, and
\item the error for a single time step is of order $\mathcal{O}\left(h^4\right)$
for timesteps of length $h$.
\end{enumerate}

The integration of the equations of motion is carried out in a
velocity-Verlet style 2-part algorithm, where $h= \delta t$:

{\tt moveA:}
\begin{align*}
{\bf v}\left(t + h / 2\right)  &\leftarrow  {\bf v}(t) 
        + \frac{h}{2} \left( {\bf f}(t) / m \right), \\
%
{\bf r}(t + h) &\leftarrow {\bf r}(t) 
        + h  {\bf v}\left(t + h / 2 \right), \\
%
{\bf j}\left(t + h / 2 \right)  &\leftarrow {\bf j}(t) 
        + \frac{h}{2} {\bf \tau}^b(t), \\
%
\mathsf{A}(t + h) &\leftarrow \mathrm{rotate}\left( h {\bf j}
        (t + h / 2) \cdot \overleftrightarrow{\mathsf{I}}^{-1} \right).
\end{align*}

In this context, the $\mathrm{rotate}$ function is the reversible product
of the three body-fixed rotations,
\begin{equation}
\mathrm{rotate}({\bf a}) = \mathsf{G}_x(a_x / 2) \cdot
\mathsf{G}_y(a_y / 2) \cdot \mathsf{G}_z(a_z) \cdot \mathsf{G}_y(a_y /
2) \cdot \mathsf{G}_x(a_x /2),
\end{equation}
where each rotational propagator, $\mathsf{G}_\alpha(\theta)$, rotates
both the rotation matrix ($\mathsf{A}$) and the body-fixed angular
momentum (${\bf j}$) by an angle $\theta$ around body-fixed axis
$\alpha$,
\begin{equation}
\mathsf{G}_\alpha( \theta ) = \left\{
\begin{array}{lcl}
\mathsf{A}(t) & \leftarrow & \mathsf{A}(0) \cdot \mathsf{R}_\alpha(\theta)^T, \\
{\bf j}(t) & \leftarrow & \mathsf{R}_\alpha(\theta) \cdot {\bf j}(0).
\end{array}
\right.
\end{equation}
$\mathsf{R}_\alpha$ is a quadratic approximation to
the single-axis rotation matrix.  For example, in the small-angle
limit, the rotation matrix around the body-fixed x-axis can be
approximated as
\begin{equation}
\mathsf{R}_x(\theta) \approx \left(
\begin{array}{ccc}
1 & 0 & 0 \\
0 & \frac{1-\theta^2 / 4}{1 + \theta^2 / 4}  & -\frac{\theta}{1+
\theta^2 / 4} \\
0 & \frac{\theta}{1+
\theta^2 / 4} & \frac{1-\theta^2 / 4}{1 + \theta^2 / 4}
\end{array}
\right).
\end{equation}
All other rotations follow in a straightforward manner.

After the first part of the propagation, the forces and body-fixed
torques are calculated at the new positions and orientations

{\tt doForces:}
\begin{align*}
{\bf f}(t + h) &\leftarrow  
        - \left(\frac{\partial V}{\partial {\bf r}}\right)_{{\bf r}(t + h)}, \\
%
{\bf \tau}^{s}(t + h) &\leftarrow {\bf u}(t + h)
        \times \frac{\partial V}{\partial {\bf u}}, \\
%
{\bf \tau}^{b}(t + h) &\leftarrow \mathsf{A}(t + h)
        \cdot {\bf \tau}^s(t + h).
\end{align*}

{\sc oopse} automatically updates ${\bf u}$ when the rotation matrix
$\mathsf{A}$ is calculated in {\tt moveA}.  Once the forces and
torques have been obtained at the new time step, the velocities can be
advanced to the same time value.

{\tt moveB:}
\begin{align*}
{\bf v}\left(t + h \right)  &\leftarrow  {\bf v}\left(t + h / 2 \right) 
        + \frac{h}{2} \left( {\bf f}(t + h) / m \right), \\
%
{\bf j}\left(t + h \right)  &\leftarrow {\bf j}\left(t + h / 2 \right) 
        + \frac{h}{2} {\bf \tau}^b(t + h) .
\end{align*}

The matrix rotations used in the {\sc dlm} method end up being more
costly computationally than the simpler arithmetic quaternion
propagation. With the same time step, a 1024-molecule water simulation
incurs an average 12\% increase in computation time using the {\sc
dlm} method in place of quaternions. This cost is more than justified
when comparing the energy conservation achieved by the two
methods. Figure ~\ref{quatdlm} provides a comparative analysis of the
{\sc dlm} method versus the traditional quaternion scheme.

\begin{figure}
\centering
\includegraphics[width=\linewidth]{quatvsdlm.eps}
\caption[Energy conservation analysis of the {\sc dlm} and quaternion 
integration methods]{Analysis of the energy conservation of the {\sc
dlm} and quaternion integration methods.  $\delta \mathrm{E}_1$ is the
linear drift in energy over time and $\delta \mathrm{E}_0$ is the
standard deviation of energy fluctuations around this drift.  All
simulations were of a 1024-molecule simulation of SSD water at 298 K
starting from the same initial configuration. Note that the {\sc dlm}
method provides more than an order of magnitude improvement in both
the energy drift and the size of the energy fluctuations when compared
with the quaternion method at any given time step.  At time steps
larger than 4 fs, the quaternion scheme resulted in rapidly rising
energies which eventually lead to simulation failure.  Using the {\sc
dlm} method, time steps up to 8 fs can be taken before this behavior
is evident.}
\label{quatdlm}
\end{figure}

In Fig.~\ref{quatdlm}, $\delta \mbox{E}_1$ is a measure of the linear
energy drift in units of $\mbox{kcal mol}^{-1}$ per particle over a
nanosecond of simulation time, and $\delta \mbox{E}_0$ is the standard
deviation of the energy fluctuations in units of $\mbox{kcal
mol}^{-1}$ per particle. In the top plot, it is apparent that the
energy drift is reduced by a significant amount (2 to 3 orders of
magnitude improvement at all tested time steps) by chosing the {\sc
dlm} method over the simple non-symplectic quaternion integration
method.  In addition to this improvement in energy drift, the
fluctuations in the total energy are also dampened by 1 to 2 orders of
magnitude by utilizing the {\sc dlm} method.

Although the {\sc dlm} method is more computationally expensive than
the traditional quaternion scheme for propagating a single time step,
consideration of the computational cost for a long simulation with a
particular level of energy conservation is in order.  A plot of energy
drift versus computational cost was generated
(Fig.~\ref{cpuCost}). This figure provides an estimate of the CPU time
required under the two integration schemes for 1 nanosecond of
simulation time for the model 1024-molecule system.  By chosing a
desired energy drift value it is possible to determine the CPU time
required for both methods. If a $\delta \mbox{E}_1$ of $1 \times
10^{-3} \mbox{kcal mol}^{-1}$ per particle is desired, a nanosecond of
simulation time will require ~19 hours of CPU time with the {\sc dlm}
integrator, while the quaternion scheme will require ~154 hours of CPU
time. This demonstrates the computational advantage of the integration
scheme utilized in {\sc oopse}.

\begin{figure}
\centering
\includegraphics[width=\linewidth]{compCost.eps}
\caption[Energy drift as a function of required simulation run 
time]{Energy drift as a function of required simulation run time.
$\delta \mathrm{E}_1$ is the linear drift in energy over time.
Simulations were performed on a single 2.5 GHz Pentium 4
processor. Simulation time comparisons can be made by tracing
horizontally from one curve to the other. For example, a simulation
that takes ~24 hours using the {\sc dlm} method will take roughly 210
hours using the simple quaternion method if the same degree of energy
conservation is desired.}
\label{cpuCost}
\end{figure}

There is only one specific keyword relevant to the default integrator,
and that is the time step for integrating the equations of motion.

\begin{center}
\begin{tabular}{llll}
{\bf variable} & {\bf Meta-data keyword} & {\bf units} & {\bf
default value} \\  
$h$ & {\tt dt = 2.0;} & fs & none 
\end{tabular}
\end{center}

\subsection{\label{sec:extended}Extended Systems for other Ensembles}

{\sc oopse} implements a number of extended system integrators for
sampling from other ensembles relevant to chemical physics.  The
integrator can be selected with the {\tt ensemble} keyword in the
meta-data file:

\begin{center}
\begin{tabular}{lll}
{\bf Integrator} & {\bf Ensemble} & {\bf Meta-data instruction} \\
NVE & microcanonical & {\tt ensemble = NVE; } \\
NVT & canonical & {\tt ensemble = NVT; } \\
NPTi & isobaric-isothermal & {\tt ensemble = NPTi;} \\
  &  (with isotropic volume changes) & \\
NPTf & isobaric-isothermal & {\tt ensemble = NPTf;} \\
  & (with changes to box shape) & \\
NPTxyz & approximate isobaric-isothermal & {\tt ensemble = NPTxyz;} \\
 &  (with separate barostats on each box dimension) & \\
\end{tabular}
\end{center}

The relatively well-known Nos\'e-Hoover thermostat\cite{Hoover85} is
implemented in {\sc oopse}'s NVT integrator.  This method couples an
extra degree of freedom (the thermostat) to the kinetic energy of the
system and it has been shown to sample the canonical distribution in
the system degrees of freedom while conserving a quantity that is, to
within a constant, the Helmholtz free energy.\cite{melchionna93}

NPT algorithms attempt to maintain constant pressure in the system by
coupling the volume of the system to a barostat.  {\sc oopse} contains
three different constant pressure algorithms.  The first two, NPTi and
NPTf have been shown to conserve a quantity that is, to within a
constant, the Gibbs free energy.\cite{melchionna93} The Melchionna
modification to the Hoover barostat is implemented in both NPTi and
NPTf.  NPTi allows only isotropic changes in the simulation box, while
box {\it shape} variations are allowed in NPTf.  The NPTxyz integrator
has {\it not} been shown to sample from the isobaric-isothermal
ensemble.  It is useful, however, in that it maintains orthogonality
for the axes of the simulation box while attempting to equalize
pressure along the three perpendicular directions in the box.

Each of the extended system integrators requires additional keywords
to set target values for the thermodynamic state variables that are
being held constant.  Keywords are also required to set the
characteristic decay times for the dynamics of the extended
variables.

\begin{center}
\begin{tabular}{llll}
{\bf variable} & {\bf Meta-data instruction} & {\bf units} & {\bf
default value} \\  
$T_{\mathrm{target}}$ & {\tt targetTemperature = 300;} &  K & none \\
$P_{\mathrm{target}}$ & {\tt targetPressure = 1;} & atm & none \\
$\tau_T$ & {\tt tauThermostat = 1e3;} & fs & none \\
$\tau_B$ & {\tt tauBarostat = 5e3;} & fs  & none \\
         & {\tt resetTime = 200;} & fs & none \\
         & {\tt useInitialExtendedSystemState = true;} & logical &
true
\end{tabular}
\end{center}

Two additional keywords can be used to either clear the extended
system variables periodically ({\tt resetTime}), or to maintain the
state of the extended system variables between simulations ({\tt
useInitialExtendedSystemState}).  More details on these variables
and their use in the integrators follows below.

\subsection{\label{oopseSec:noseHooverThermo}Nos\'{e}-Hoover Thermostatting}

The Nos\'e-Hoover equations of motion are given by\cite{Hoover85}
\begin{eqnarray}
\dot{{\bf r}} & = & {\bf v}, \\
\dot{{\bf v}} & = & \frac{{\bf f}}{m} - \chi {\bf v} ,\\
\dot{\mathsf{A}} & = & \mathsf{A} \cdot
\mbox{ skew}\left(\overleftrightarrow{\mathsf{I}}^{-1} \cdot {\bf j}\right), \\
\dot{{\bf j}} & = & {\bf j} \times \left( \overleftrightarrow{\mathsf{I}}^{-1}
\cdot {\bf j} \right) - \mbox{ rot}\left(\mathsf{A}^{T} \cdot \frac{\partial
V}{\partial \mathsf{A}} \right) - \chi {\bf j}.
\label{eq:nosehoovereom}
\end{eqnarray}

$\chi$ is an ``extra'' variable included in the extended system, and
it is propagated using the first order equation of motion
\begin{equation}
\dot{\chi} = \frac{1}{\tau_{T}^2} \left( \frac{T}{T_{\mathrm{target}}} - 1 \right).
\label{eq:nosehooverext}
\end{equation}

The instantaneous temperature $T$ is proportional to the total kinetic
energy (both translational and orientational) and is given by
\begin{equation}
T = \frac{2 K}{f k_B}
\end{equation}
Here, $f$ is the total number of degrees of freedom in the system,
\begin{equation}
f = 3 N + 2 N_{\mathrm{linear}} + 3 N_{\mathrm{non-linear}} - N_{\mathrm{constraints}},
\end{equation}
and $K$ is the total kinetic energy,
\begin{equation}
K = \sum_{i=1}^{N} \frac{1}{2} m_i {\bf v}_i^T \cdot {\bf v}_i +
\sum_{i=1}^{N_{\mathrm{linear}}+N_{\mathrm{non-linear}}}  \frac{1}{2} {\bf j}_i^T \cdot
\overleftrightarrow{\mathsf{I}}_i^{-1} \cdot {\bf j}_i.
\end{equation}
$N_{\mathrm{linear}}$ is the number of linear rotors (i.e. with two
non-zero moments of inertia), and $N_{\mathrm{non-linear}}$ is the
number of non-linear rotors (i.e. with three non-zero moments of
inertia).  

In eq.(\ref{eq:nosehooverext}), $\tau_T$ is the time constant for
relaxation of the temperature to the target value.  To set values for
$\tau_T$ or $T_{\mathrm{target}}$ in a simulation, one would use the
{\tt tauThermostat} and {\tt targetTemperature} keywords in the
meta-data file.  The units for {\tt tauThermostat} are fs, and the
units for the {\tt targetTemperature} are degrees K.   The integration
of the equations of motion is carried out in a velocity-Verlet style 2
part algorithm:

{\tt moveA:}
\begin{align*}
T(t) &\leftarrow \left\{{\bf v}(t)\right\}, \left\{{\bf j}(t)\right\} ,\\
%
{\bf v}\left(t + h / 2\right)  &\leftarrow {\bf v}(t) 
        + \frac{h}{2} \left( \frac{{\bf f}(t)}{m} - {\bf v}(t)
        \chi(t)\right), \\
%
{\bf r}(t + h) &\leftarrow {\bf r}(t) 
        + h {\bf v}\left(t + h / 2 \right) ,\\
%
{\bf j}\left(t + h / 2 \right)  &\leftarrow {\bf j}(t) 
        + \frac{h}{2} \left( {\bf \tau}^b(t) - {\bf j}(t)
        \chi(t) \right) ,\\
%
\mathsf{A}(t + h) &\leftarrow \mathrm{rotate}
        \left(h * {\bf j}(t + h / 2) 
        \overleftrightarrow{\mathsf{I}}^{-1} \right) ,\\
%
\chi\left(t + h / 2 \right) &\leftarrow \chi(t) 
        + \frac{h}{2 \tau_T^2} \left( \frac{T(t)}
        {T_{\mathrm{target}}} - 1 \right) .
\end{align*}

Here $\mathrm{rotate}(h * {\bf j}
\overleftrightarrow{\mathsf{I}}^{-1})$ is the same symplectic Trotter
factorization of the three rotation operations that was discussed in
the section on the {\sc dlm} integrator.  Note that this operation modifies
both the rotation matrix $\mathsf{A}$ and the angular momentum ${\bf
j}$.  {\tt moveA} propagates velocities by a half time step, and
positional degrees of freedom by a full time step.  The new positions
(and orientations) are then used to calculate a new set of forces and
torques in exactly the same way they are calculated in the {\tt
doForces} portion of the {\sc dlm} integrator.

Once the forces and torques have been obtained at the new time step,
the temperature, velocities, and the extended system variable can be
advanced to the same time value.

{\tt moveB:}
\begin{align*}
T(t + h) &\leftarrow \left\{{\bf v}(t + h)\right\}, 
        \left\{{\bf j}(t + h)\right\}, \\
%
\chi\left(t + h \right) &\leftarrow \chi\left(t + h /
        2 \right) + \frac{h}{2 \tau_T^2} \left( \frac{T(t+h)}
        {T_{\mathrm{target}}} - 1 \right), \\
%
{\bf v}\left(t + h \right)  &\leftarrow {\bf v}\left(t 
        + h / 2 \right) + \frac{h}{2} \left(
        \frac{{\bf f}(t + h)}{m} - {\bf v}(t + h)
        \chi(t h)\right) ,\\
%
{\bf j}\left(t + h \right) &\leftarrow {\bf j}\left(t
        + h / 2 \right) + \frac{h}{2} 
        \left( {\bf \tau}^b(t + h) - {\bf j}(t + h) 
        \chi(t + h) \right) .
\end{align*}

Since ${\bf v}(t + h)$ and ${\bf j}(t + h)$ are required to calculate
$T(t + h)$ as well as $\chi(t + h)$, they indirectly depend on their
own values at time $t + h$.  {\tt moveB} is therefore done in an
iterative fashion until $\chi(t + h)$ becomes self-consistent.  The
relative tolerance for the self-consistency check defaults to a value
of $\mbox{10}^{-6}$, but {\sc oopse} will terminate the iteration
after 4 loops even if the consistency check has not been satisfied.

The Nos\'e-Hoover algorithm is known to conserve a Hamiltonian for the
extended system that is, to within a constant, identical to the
Helmholtz free energy,\cite{melchionna93}
\begin{equation}
H_{\mathrm{NVT}} = V + K + f k_B T_{\mathrm{target}} \left(
\frac{\tau_{T}^2 \chi^2(t)}{2} + \int_{0}^{t} \chi(t^\prime) dt^\prime
\right).
\end{equation}
Poor choices of $h$ or $\tau_T$ can result in non-conservation
of $H_{\mathrm{NVT}}$, so the conserved quantity is maintained in the
last column of the {\tt .stat} file to allow checks on the quality of
the integration.

Bond constraints are applied at the end of both the {\tt moveA} and
{\tt moveB} portions of the algorithm.  Details on the constraint
algorithms are given in section \ref{oopseSec:rattle}.

\subsection{\label{sec:NPTi}Constant-pressure integration with 
isotropic box deformations (NPTi)}

To carry out isobaric-isothermal ensemble calculations, {\sc oopse}
implements the Melchionna modifications to the Nos\'e-Hoover-Andersen
equations of motion.\cite{melchionna93} The equations of motion are
the same as NVT with the following exceptions:

\begin{eqnarray}
\dot{{\bf r}} & = & {\bf v} + \eta \left( {\bf r} - {\bf R}_0 \right), \\
\dot{{\bf v}} & = & \frac{{\bf f}}{m} - (\eta + \chi) {\bf v}, \\
\dot{\eta} & = & \frac{1}{\tau_{B}^2 f k_B T_{\mathrm{target}}} V \left( P -
P_{\mathrm{target}} \right), \\
\dot{\mathcal{V}} & = & 3 \mathcal{V} \eta .
\label{eq:melchionna1}
\end{eqnarray}

$\chi$ and $\eta$ are the ``extra'' degrees of freedom in the extended
system.  $\chi$ is a thermostat, and it has the same function as it
does in the Nos\'e-Hoover NVT integrator.  $\eta$ is a barostat which
controls changes to the volume of the simulation box.  ${\bf R}_0$ is
the location of the center of mass for the entire system, and
$\mathcal{V}$ is the volume of the simulation box.  At any time, the
volume can be calculated from the determinant of the matrix which
describes the box shape:
\begin{equation}
\mathcal{V} = \det(\mathsf{H}).
\end{equation}

The NPTi integrator requires an instantaneous pressure. This quantity
is calculated via the pressure tensor,
\begin{equation}
\overleftrightarrow{\mathsf{P}}(t) = \frac{1}{\mathcal{V}(t)} \left(
\sum_{i=1}^{N} m_i {\bf v}_i(t) \otimes {\bf v}_i(t) \right) +
\overleftrightarrow{\mathsf{W}}(t).
\end{equation}
The kinetic contribution to the pressure tensor utilizes the {\it
outer} product of the velocities, denoted by the $\otimes$ symbol.  The
stress tensor is calculated from another outer product of the
inter-atomic separation vectors (${\bf r}_{ij} = {\bf r}_j - {\bf
r}_i$) with the forces between the same two atoms,
\begin{equation}
\overleftrightarrow{\mathsf{W}}(t) = \sum_{i} \sum_{j>i} {\bf r}_{ij}(t)
\otimes {\bf f}_{ij}(t).
\end{equation}
In systems containing cutoff groups, the stress tensor is computed
between the centers-of-mass of the cutoff groups:
\begin{equation}
\overleftrightarrow{\mathsf{W}}(t) = \sum_{a} \sum_{b} {\bf r}_{ab}(t)
\otimes {\bf f}_{ab}(t).
\end{equation}
where ${\bf r}_{ab}$ is the distance between the centers of mass, and 
\begin{equation}
{\bf f}_{ab} = s(r_{ab}) \sum_{i \in a} \sum_{j \in b} {\bf f}_{ij} +
s^{\prime}(r_{ab}) \frac{{\bf r}_{ab}}{|r_{ab}|} \sum_{i \in a} \sum_{j
\in b} V_{ij}({\bf r}_{ij}).
\end{equation}

The instantaneous pressure is then simply obtained from the trace of
the pressure tensor,
\begin{equation}
P(t) = \frac{1}{3} \mathrm{Tr} \left( \overleftrightarrow{\mathsf{P}}(t)
\right).
\end{equation}

In eq.(\ref{eq:melchionna1}), $\tau_B$ is the time constant for
relaxation of the pressure to the target value.  To set values for
$\tau_B$ or $P_{\mathrm{target}}$ in a simulation, one would use the
{\tt tauBarostat} and {\tt targetPressure} keywords in the meta-data
file.  The units for {\tt tauBarostat} are fs, and the units for the
{\tt targetPressure} are atmospheres.  Like in the NVT integrator, the
integration of the equations of motion is carried out in a
velocity-Verlet style two part algorithm with only the following
differences:

{\tt moveA:}
\begin{align*}
P(t) &\leftarrow \left\{{\bf r}(t)\right\}, \left\{{\bf v}(t)\right\} ,\\
%
{\bf v}\left(t + h / 2\right)  &\leftarrow {\bf v}(t) 
        + \frac{h}{2} \left( \frac{{\bf f}(t)}{m} - {\bf v}(t)
        \left(\chi(t) + \eta(t) \right) \right), \\
%
\eta(t + h / 2) &\leftarrow \eta(t) + \frac{h 
        \mathcal{V}(t)}{2 N k_B T(t) \tau_B^2} \left( P(t) 
        - P_{\mathrm{target}} \right), \\ 
%
{\bf r}(t + h) &\leftarrow {\bf r}(t) + h 
        \left\{ {\bf v}\left(t + h / 2 \right) 
        + \eta(t + h / 2)\left[ {\bf r}(t + h) 
        - {\bf R}_0 \right] \right\} ,\\
%
\mathsf{H}(t + h) &\leftarrow e^{-h \eta(t + h / 2)} 
        \mathsf{H}(t).
\end{align*}

The propagation of positions to time $t + h$
depends on the positions at the same time.  {\sc oopse} carries out
this step iteratively (with a limit of 5 passes through the iterative
loop).  Also, the simulation box $\mathsf{H}$ is scaled uniformly for
one full time step by an exponential factor that depends on the value
of $\eta$ at time $t +
h / 2$.  Reshaping the box uniformly also scales the volume of
the box by
\begin{equation}
\mathcal{V}(t + h) \leftarrow e^{ - 3 h \eta(t + h /2)} \times
\mathcal{V}(t).
\end{equation}

The {\tt doForces} step for the NPTi integrator is exactly the same as
in both the {\sc dlm} and NVT integrators.  Once the forces and torques have
been obtained at the new time step, the velocities can be advanced to
the same time value.

{\tt moveB:}
\begin{align*}
P(t + h) &\leftarrow  \left\{{\bf r}(t + h)\right\},
        \left\{{\bf v}(t + h)\right\}, \\
%
\eta(t + h) &\leftarrow \eta(t + h / 2) +
        \frac{h \mathcal{V}(t + h)}{2 N k_B T(t + h) 
        \tau_B^2} \left( P(t + h) - P_{\mathrm{target}} \right), \\ 
%
{\bf v}\left(t + h \right)  &\leftarrow {\bf v}\left(t 
        + h / 2 \right) + \frac{h}{2} \left(
        \frac{{\bf f}(t + h)}{m} - {\bf v}(t + h)
        (\chi(t + h) + \eta(t + h)) \right) ,\\
%
{\bf j}\left(t + h \right)  &\leftarrow {\bf j}\left(t 
        + h / 2 \right) + \frac{h}{2} \left( {\bf
        \tau}^b(t + h) - {\bf j}(t + h)
        \chi(t + h) \right) .
\end{align*}

Once again, since ${\bf v}(t + h)$ and ${\bf j}(t + h)$ are required
to calculate $T(t + h)$, $P(t + h)$, $\chi(t + h)$, and $\eta(t +
h)$, they indirectly depend on their own values at time $t + h$.  {\tt
moveB} is therefore done in an iterative fashion until $\chi(t + h)$
and $\eta(t + h)$ become self-consistent.  The relative tolerance for
the self-consistency check defaults to a value of $\mbox{10}^{-6}$,
but {\sc oopse} will terminate the iteration after 4 loops even if the
consistency check has not been satisfied.

The Melchionna modification of the Nos\'e-Hoover-Andersen algorithm is
known to conserve a Hamiltonian for the extended system that is, to
within a constant, identical to the Gibbs free energy,
\begin{equation}
H_{\mathrm{NPTi}} = V + K + f k_B T_{\mathrm{target}} \left(
\frac{\tau_{T}^2 \chi^2(t)}{2} + \int_{0}^{t} \chi(t^\prime) dt^\prime
\right) + P_{\mathrm{target}} \mathcal{V}(t).
\end{equation}
Poor choices of $\delta t$, $\tau_T$, or $\tau_B$ can result in
non-conservation of $H_{\mathrm{NPTi}}$, so the conserved quantity is
maintained in the last column of the {\tt .stat} file to allow checks
on the quality of the integration.  It is also known that this
algorithm samples the equilibrium distribution for the enthalpy
(including contributions for the thermostat and barostat), 
\begin{equation}
H_{\mathrm{NPTi}} = V + K + \frac{f k_B T_{\mathrm{target}}}{2} \left(
\chi^2 \tau_T^2 + \eta^2 \tau_B^2 \right) +  P_{\mathrm{target}}
\mathcal{V}(t). 
\end{equation}

Bond constraints are applied at the end of both the {\tt moveA} and
{\tt moveB} portions of the algorithm.  Details on the constraint
algorithms are given in section \ref{oopseSec:rattle}.

\subsection{\label{sec:NPTf}Constant-pressure integration with a
flexible box (NPTf)} 

There is a relatively simple generalization of the
Nos\'e-Hoover-Andersen method to include changes in the simulation box
{\it shape} as well as in the volume of the box.  This method utilizes
the full $3 \times 3$ pressure tensor and introduces a tensor of
extended variables ($\overleftrightarrow{\eta}$) to control changes to
the box shape.  The equations of motion for this method differ from
those of NPTi as follows:
\begin{eqnarray}
\dot{{\bf r}} & = & {\bf v} + \overleftrightarrow{\eta} \cdot \left( {\bf r} - {\bf R}_0 \right), \\
\dot{{\bf v}} & = & \frac{{\bf f}}{m} - (\overleftrightarrow{\eta} +
\chi \cdot \mathsf{1}) {\bf v}, \\
\dot{\overleftrightarrow{\eta}} & = & \frac{1}{\tau_{B}^2 f k_B
T_{\mathrm{target}}} V \left( \overleftrightarrow{\mathsf{P}} - P_{\mathrm{target}}\mathsf{1} \right) ,\\
\dot{\mathsf{H}} & = &  \overleftrightarrow{\eta} \cdot \mathsf{H} .
\label{eq:melchionna2}
\end{eqnarray}

Here, $\mathsf{1}$ is the unit matrix and $\overleftrightarrow{\mathsf{P}}$
is the pressure tensor.  Again, the volume, $\mathcal{V} = \det
\mathsf{H}$. 

The propagation of the equations of motion is nearly identical to the
NPTi integration:

{\tt moveA:}
\begin{align*}
\overleftrightarrow{\mathsf{P}}(t) &\leftarrow \left\{{\bf r}(t)\right\}, 
        \left\{{\bf v}(t)\right\} ,\\
%
{\bf v}\left(t + h / 2\right)  &\leftarrow {\bf v}(t) 
        + \frac{h}{2} \left( \frac{{\bf f}(t)}{m} - 
        \left(\chi(t)\mathsf{1} + \overleftrightarrow{\eta}(t) \right) \cdot
        {\bf v}(t) \right), \\ 
%
\overleftrightarrow{\eta}(t + h / 2) &\leftarrow 
        \overleftrightarrow{\eta}(t) + \frac{h \mathcal{V}(t)}{2 N k_B
        T(t) \tau_B^2} \left( \overleftrightarrow{\mathsf{P}}(t) 
        - P_{\mathrm{target}}\mathsf{1} \right), \\ 
%
{\bf r}(t + h) &\leftarrow {\bf r}(t) + h \left\{ {\bf v}
        \left(t + h / 2 \right) + \overleftrightarrow{\eta}(t +
        h / 2) \cdot \left[ {\bf r}(t + h) 
        - {\bf R}_0 \right] \right\}, \\
%
\mathsf{H}(t + h) &\leftarrow \mathsf{H}(t) \cdot e^{-h
        \overleftrightarrow{\eta}(t + h / 2)} .
\end{align*}
{\sc oopse} uses a power series expansion truncated at second order
for the exponential operation which scales the simulation box.

The {\tt moveB} portion of the algorithm is largely unchanged from the
NPTi integrator:

{\tt moveB:}
\begin{align*}
\overleftrightarrow{\mathsf{P}}(t + h) &\leftarrow \left\{{\bf r}
        (t + h)\right\}, \left\{{\bf v}(t 
        + h)\right\}, \left\{{\bf f}(t + h)\right\} ,\\
%
\overleftrightarrow{\eta}(t + h) &\leftarrow 
        \overleftrightarrow{\eta}(t + h / 2) +
        \frac{h \mathcal{V}(t + h)}{2 N k_B T(t + h) 
        \tau_B^2} \left( \overleftrightarrow{P}(t + h) 
        - P_{\mathrm{target}}\mathsf{1} \right) ,\\ 
%
{\bf v}\left(t + h \right)  &\leftarrow {\bf v}\left(t 
        + h / 2 \right) + \frac{h}{2} \left(
        \frac{{\bf f}(t + h)}{m} - 
        (\chi(t + h)\mathsf{1} + \overleftrightarrow{\eta}(t 
        + h)) \right) \cdot {\bf v}(t + h), \\
\end{align*}

The iterative schemes for both {\tt moveA} and {\tt moveB} are
identical to those described for the NPTi integrator.

The NPTf integrator is known to conserve the following Hamiltonian:
\begin{equation}
H_{\mathrm{NPTf}} = V + K + f k_B T_{\mathrm{target}} \left(
\frac{\tau_{T}^2 \chi^2(t)}{2} + \int_{0}^{t} \chi(t^\prime) dt^\prime
\right) + P_{\mathrm{target}} \mathcal{V}(t) + \frac{f k_B
T_{\mathrm{target}}}{2}
\mathrm{Tr}\left[\overleftrightarrow{\eta}(t)\right]^2 \tau_B^2.
\end{equation}

This integrator must be used with care, particularly in liquid
simulations.  Liquids have very small restoring forces in the
off-diagonal directions, and the simulation box can very quickly form
elongated and sheared geometries which become smaller than the cutoff
radius.  The NPTf integrator finds most use in simulating crystals or
liquid crystals which assume non-orthorhombic geometries.

\subsection{\label{nptxyz}Constant pressure in 3 axes (NPTxyz)}

There is one additional extended system integrator which is somewhat
simpler than the NPTf method described above.  In this case, the three
axes have independent barostats which each attempt to preserve the
target pressure along the box walls perpendicular to that particular
axis.  The lengths of the box axes are allowed to fluctuate
independently, but the angle between the box axes does not change.
The equations of motion are identical to those described above, but
only the {\it diagonal} elements of $\overleftrightarrow{\eta}$ are
computed.  The off-diagonal elements are set to zero (even when the
pressure tensor has non-zero off-diagonal elements).

It should be noted that the NPTxyz integrator is {\it not} known to
preserve any Hamiltonian of interest to the chemical physics
community.  The integrator is extremely useful, however, in generating
initial conditions for other integration methods.  It {\it is} suitable
for use with liquid simulations, or in cases where there is
orientational anisotropy in the system (i.e. in lipid bilayer
simulations).

\subsection{\label{sec:constraints}Constraint Methods}

\subsubsection{\label{oopseSec:rattle}The {\sc rattle} Method for Bond 
        Constraints}

In order to satisfy the constraints of fixed bond lengths within {\sc
oopse}, we have implemented the {\sc rattle} algorithm of
Andersen.\cite{andersen83} {\sc rattle} is a velocity-Verlet
formulation of the {\sc shake} method\cite{ryckaert77} for iteratively
solving the Lagrange multipliers which maintain the holonomic
constraints.  Both methods are covered in depth in the
literature,\cite{leach01:mm,allen87:csl} and a detailed description of
this method would be redundant.

\subsubsection{\label{oopseSec:zcons}The Z-Constraint Method}

A force auto-correlation method based on the fluctuation-dissipation
theorem was developed by Roux and Karplus to investigate the dynamics
of ions inside ion channels.\cite{Roux91} The time-dependent friction
coefficient can be calculated from the deviation of the instantaneous
force from its mean value:
\begin{equation}
\xi(z,t)=\langle\delta F(z,t)\delta F(z,0)\rangle/k_{B}T,
\end{equation}
where%
\begin{equation}
\delta F(z,t)=F(z,t)-\langle F(z,t)\rangle.
\end{equation}

If the time-dependent friction decays rapidly, the static friction
coefficient can be approximated by
\begin{equation}
\xi_{\text{static}}(z)=\int_{0}^{\infty}\langle\delta F(z,t)\delta F(z,0)\rangle dt.
\end{equation}

This allows the diffusion constant to then be calculated through the
Einstein relation:\cite{Marrink94}
\begin{equation}
D(z)=\frac{k_{B}T}{\xi_{\text{static}}(z)}=\frac{(k_{B}T)^{2}}{\int_{0}^{\infty
}\langle\delta F(z,t)\delta F(z,0)\rangle dt}.%
\end{equation}

The Z-Constraint method, which fixes the $z$ coordinates of a few
``tagged'' molecules with respect to the center of the mass of the
system is a technique that was proposed to obtain the forces required
for the force auto-correlation calculation.\cite{Marrink94} However,
simply resetting the coordinate will move the center of the mass of
the whole system. To avoid this problem, we have developed a new
method that is utilized in {\sc oopse}. Instead of resetting the
coordinates, we reset the forces of $z$-constrained molecules and
subtract the total constraint forces from the rest of the system after
the force calculation at each time step.

After the force calculation, the total force on molecule $\alpha$ is:
\begin{equation}
G_{\alpha} = \sum_i F_{\alpha i},
\label{oopseEq:zc1}
\end{equation}
where $F_{\alpha i}$ is the force in the $z$ direction on atom $i$ in
$z$-constrained molecule $\alpha$. The forces on the atoms in the
$z$-constrained molecule are then adjusted to remove the total force
on molecule $\alpha$:
\begin{equation}
F_{\alpha i} = F_{\alpha i} - 
        \frac{m_{\alpha i} G_{\alpha}}{\sum_i m_{\alpha i}}.
\end{equation}
Here, $m_{\alpha i}$ is the mass of atom $i$ in the $z$-constrained
molecule.  After the forces have been adjusted, the velocities must
also be modified to subtract out molecule $\alpha$'s center-of-mass
velocity in the $z$ direction.
\begin{equation}
v_{\alpha i} = v_{\alpha i} -
        \frac{\sum_i m_{\alpha i} v_{\alpha i}}{\sum_i m_{\alpha i}},
\end{equation}
where $v_{\alpha i}$ is the velocity of atom $i$ in the $z$ direction.
Lastly, all of the accumulated constraint forces must be subtracted
from the rest of the unconstrained system to keep the system center of
mass of the entire system from drifting.
\begin{equation}
F_{\beta i} = F_{\beta i} - \frac{m_{\beta i} \sum_{\alpha} G_{\alpha}}
        {\sum_{\beta}\sum_i m_{\beta i}},
\end{equation}
where $\beta$ denotes all {\it unconstrained} molecules in the
system. Similarly, the velocities of the unconstrained molecules must
also be scaled:
\begin{equation}
v_{\beta i} = v_{\beta i} + \sum_{\alpha} \frac{\sum_i m_{\alpha i}
v_{\alpha i}}{\sum_i m_{\alpha i}}.
\end{equation}

This method will pin down the centers-of-mass of all of the
$z$-constrained molecules, and will also keep the entire system fixed
at the original system center-of-mass location.

At the very beginning of the simulation, the molecules may not be at
their desired positions. To steer a $z$-constrained molecule to its
specified position, a simple harmonic potential is used:
\begin{equation}
U(t)=\frac{1}{2}k_{\text{Harmonic}}(z(t)-z_{\text{cons}})^{2},%
\end{equation}
where $k_{\text{Harmonic}}$ is an harmonic force constant, $z(t)$ is
the current $z$ coordinate of the center of mass of the constrained
molecule, and $z_{\text{cons}}$ is the desired constraint
position. The harmonic force operating on the $z$-constrained molecule
at time $t$ can be calculated by
\begin{equation}
F_{z_{\text{Harmonic}}}(t)=-\frac{\partial U(t)}{\partial z(t)}=
        -k_{\text{Harmonic}}(z(t)-z_{\text{cons}}).
\end{equation}

The user may also specify the use of a constant velocity method
(steered molecular dynamics) to move the molecules to their desired
initial positions. Based on concepts from atomic force microscopy,
{\sc smd} has been used to study many processes which occur via rare
events on the time scale of a few hundreds of picoseconds.  For
example,{\sc smd} has been used to observe the dissociation of
Streptavidin-biotin Complex.\cite{smd}  

To use of the $z$-constraint method in an {\sc oopse} simulation, the
molecules must be specified using the {\tt nZconstraints} keyword in
the meta-data file.  The other parameters for modifying the behavior
of the $z$-constraint method are listed in table~\ref{table:zconParams}.

\begin{table}
\caption{Meta-data Keywords: Z-Constraint Parameters}
\label{table:zconParams}
\begin{center}
% Note when adding or removing columns, the \hsize numbers must add up to the total number
% of columns.
\begin{tabularx}{\linewidth}%
  {>{\setlength{\hsize}{1.00\hsize}}X%
  >{\setlength{\hsize}{0.4\hsize}}X%
  >{\setlength{\hsize}{1.2\hsize}}X%
  >{\setlength{\hsize}{1.4\hsize}}X}

{\bf keyword} & {\bf units} & {\bf use} & {\bf remarks} \\ \hline

{\tt nZconstraints} & integer &  The number of $z$-constrained
molecules & If using the $z$-constraint method, {\tt nZconstraints}
must be set \\ 
{\tt zconsTime} & fs & Sets the frequency at which the {\tt .fz} file
is written &  \\ 
{\tt zconsForcePolicy} & string & The strategy for subtracting
the $z$-constraint force from the {\it unconstrained} molecules & Possible
strategies are {\tt BYMASS} and {\tt BYNUMBER}. The default
strategy is {\tt BYMASS}\\ 
{\tt zconsGap} & $\mbox{\AA}$ & Sets the distance between two adjacent
constraint positions&Used mainly to move molecules through a
simulation to estimate potentials of mean force. \\ 
{\tt zconsFixtime} & fs & Sets the length of time the $z$-constraint
molecule is held fixed & {\tt zconsFixtime} must be set if {\tt
zconsGap} is set\\ 
{\tt zconsUsingSMD} & logical & Flag for using Steered Molecular
Dynamics to move the molecules to the correct constrained positions  &
Harmonic Forces are used by default\\

\end{tabularx}
\end{center}
\end{table}


\section{\label{oopseSec:minimizer}Energy Minimization}

As one of the basic procedures of molecular modeling, energy
minimization is used to identify local configurations that are stable
points on the potential energy surface. There is a vast literature on
energy minimization algorithms have been developed to search for the
global energy minimum as well as to find local structures which are
stable fixed points on the surface.  We have included two simple
minimization algorithms: steepest descent, ({\sc sd}) and conjugate
gradient ({\sc cg}) to help users find reasonable local minima from
their initial configurations. Since {\sc oopse} handles atoms and
rigid bodies which have orientational coordinates as well as
translational coordinates, there is some subtlety to the choice of
parameters for minimization algorithms.

Given a coordinate set $x_{k}$ and a search direction $d_{k}$, a line
search algorithm is performed along $d_{k}$ to produce
$x_{k+1}=x_{k}+$ $\lambda _{k}d_{k}$. In the steepest descent ({\sc
sd}) algorithm,%
\begin{equation}
d_{k}=-\nabla V(x_{k}).
\end{equation}
The gradient and the direction of next step are always orthogonal.
This may cause oscillatory behavior in narrow valleys.  To overcome
this problem, the Fletcher-Reeves variant~\cite{FletcherReeves} of the
conjugate gradient ({\sc cg}) algorithm is used to generate $d_{k+1}$
via simple recursion:
\begin{equation}
d_{k+1}  =-\nabla V(x_{k+1})+\gamma_{k}d_{k}
\end{equation}
where
\begin{equation}
\gamma_{k}  =\frac{\nabla V(x_{k+1})^{T}\nabla V(x_{k+1})}{\nabla
V(x_{k})^{T}\nabla V(x_{k})}.
\end{equation}

The Polak-Ribiere variant~\cite{PolakRibiere} of the conjugate
gradient ($\gamma_{k}$) is defined as%
\begin{equation}
\gamma_{k}=\frac{[\nabla V(x_{k+1})-\nabla V(x)]^{T}\nabla V(x_{k+1})}{\nabla
V(x_{k})^{T}\nabla V(x_{k})}%
\end{equation}
It is widely agreed that the Polak-Ribiere variant gives better
convergence than the Fletcher-Reeves variant, so the conjugate
gradient approach implemented in {\sc oopse} is the Polak-Ribiere
variant.

The conjugate gradient method assumes that the conformation is close
enough to a local minimum that the potential energy surface is very
nearly quadratic.  When the initial structure is far from the minimum,
the steepest descent method can be superior to the conjugate gradient
method. Hence, the steepest descent method is often used for the first
10-100 steps of minimization. Another useful feature of minimization
methods in {\sc oopse} is that a modified {\sc shake} algorithm can be
applied during the minimization to constraint the bond lengths if this
is required by the force field. Meta-data parameters concerning the
minimizer are given in Table~\ref{table:minimizeParams}

\begin{table}
\caption{Meta-data Keywords: Energy Minimizer Parameters}
\label{table:minimizeParams}
\begin{center}
% Note when adding or removing columns, the \hsize numbers must add up to the total number
% of columns.
\begin{tabularx}{\linewidth}%
  {>{\setlength{\hsize}{1.2\hsize}}X%
  >{\setlength{\hsize}{0.6\hsize}}X%
  >{\setlength{\hsize}{1.1\hsize}}X%
  >{\setlength{\hsize}{1.1\hsize}}X}

{\bf keyword} & {\bf units} & {\bf use} & {\bf remarks} \\ \hline

{\tt minimizer} & string &  selects the minimization method to be used
& either {\tt CG} (conjugate gradient) or {\tt SD} (steepest
descent) \\ 
{\tt minimizerMaxIter} & steps & Sets the maximum number of iterations
for the energy minimization & The default value is 200\\
{\tt minimizerWriteFreq} & steps & Sets the frequency with which the {\tt .dump} and {\tt .stat} files are writtern during energy minimization & \\
{\tt minimizerStepSize} & $\mbox{\AA}$ &  Sets the step size for the
line search & The default value is 0.01\\
{\tt minimizerFTol} & $\mbox{kcal mol}^{-1}$  & Sets the energy tolerance
for stopping the minimziation. & The default value is $10^{-8}$\\
{\tt minimizerGTol} & $\mbox{kcal mol}^{-1}\mbox{\AA}^{-1}$ & Sets the
gradient tolerance for stopping the minimization. & The default value
is  $10^{-8}$\\ 
{\tt minimizerLSTol} &  $\mbox{kcal mol}^{-1}$ & Sets line search
tolerance for terminating each step of the minimization. & The default 
value is $10^{-8}$\\ 
{\tt minimizerLSMaxIter} & steps &  Sets the maximum number of
iterations for each line search & The default value is 50\\

\end{tabularx}
\end{center}
\end{table}

\section{\label{oopseSec:parallelization} Parallel Simulation Implementation}

Although processor power is continually improving, it is still
unreasonable to simulate systems of more than 10,000 atoms on a single
processor. To facilitate study of larger system sizes or smaller
systems for longer time scales, parallel methods were developed to
allow multiple CPU's to share the simulation workload. Three general
categories of parallel decomposition methods have been developed:
these are the atomic,\cite{Fox88} spatial~\cite{plimpton95} and
force~\cite{Paradyn} decomposition methods.

Algorithmically simplest of the three methods is atomic decomposition,
where $N$ particles in a simulation are split among $P$ processors for
the duration of the simulation. Computational cost scales as an
optimal $\mathcal{O}(N/P)$ for atomic decomposition. Unfortunately, all
processors must communicate positions and forces with all other
processors at every force evaluation, leading the communication costs
to scale as an unfavorable $\mathcal{O}(N)$, \emph{independent of the
number of processors}. This communication bottleneck led to the
development of spatial and force decomposition methods, in which
communication among processors scales much more favorably. Spatial or
domain decomposition divides the physical spatial domain into 3D boxes
in which each processor is responsible for calculation of forces and
positions of particles located in its box. Particles are reassigned to
different processors as they move through simulation space. To
calculate forces on a given particle, a processor must simply know the
positions of particles within some cutoff radius located on nearby
processors rather than the positions of particles on all
processors. Both communication between processors and computation
scale as $\mathcal{O}(N/P)$ in the spatial method. However, spatial
decomposition adds algorithmic complexity to the simulation code and
is not very efficient for small $N$, since the overall communication
scales as the surface to volume ratio $\mathcal{O}(N/P)^{2/3}$ in
three dimensions.

The parallelization method used in {\sc oopse} is the force
decomposition method.\cite{hendrickson:95} Force decomposition assigns
particles to processors based on a block decomposition of the force
matrix. Processors are split into an optimally square grid forming row
and column processor groups. Forces are calculated on particles in a
given row by particles located in that processor's column
assignment. One deviation from the algorithm described by Hendrickson
{\it et al.} is the use of column ordering based on the row indexes
preventing the need for a transpose operation necessitating a second
communication step when gathering the final force components.  Force
decomposition is less complex to implement than the spatial method but
still scales computationally as $\mathcal{O}(N/P)$ and scales as
$\mathcal{O}(N/\sqrt{P})$ in communication cost. Plimpton has also
found that force decompositions scale more favorably than spatial
decompositions for systems up to 10,000 atoms and favorably compete
with spatial methods up to 100,000 atoms.\cite{plimpton95}
 
\section{\label{oopseSec:conclusion}Conclusion}

We have presented a new parallel simulation program called {\sc
oopse}. This program offers some novel capabilities, but mostly makes
available a library of modern object-oriented code for the scientific
community to use freely.  Notably, {\sc oopse} can handle symplectic
integration of objects (atoms and rigid bodies) which have
orientational degrees of freedom.  It can also work with transition
metal force fields and point-dipoles. It is capable of scaling across
multiple processors through the use of force based decomposition. It
also implements several advanced integrators allowing the end user
control over temperature and pressure. In addition, it is capable of
integrating constrained dynamics through both the {\sc rattle}
algorithm and the $z$-constraint method.

We encourage other researchers to download and apply this program to
their own research problems.  By making the code available, we hope to
encourage other researchers to contribute their own code and make it a
more powerful package for everyone in the molecular dynamics community
to use.  All source code for {\sc oopse} is available for download at
{\tt http://oopse.org}.

\newpage
\section{Acknowledgments}

Development of {\sc oopse} was funded by a New Faculty Award from the
Camille and Henry Dreyfus Foundation and by the National Science
Foundation under grant CHE-0134881. Computation time was provided by
the Notre Dame Bunch-of-Boxes (B.o.B) computer cluster under NSF grant
DMR-0079647.

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Revision:	1439
Committed:	Thu Jul 29 20:06:07 2004 UTC (20 years, 1 month ago) by gezelter
Content type:	application/x-tex
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Log Message:	Post-submission clean-up