multipole/multipole_2/multipole2.tex

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\title{Real space electrostatics for multipoles. II. Comparisons with
  the Ewald Sum}

\author{Madan Lamichhane}
 \affiliation{Department of Physics, University of Notre Dame, Notre Dame, IN 46556}

\author{Kathie E. Newman}
\affiliation{Department of Physics, University of Notre Dame, Notre Dame, IN 46556}

\author{J. Daniel Gezelter}%
 \email{gezelter@nd.edu.}
\affiliation{Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556
}

\date{\today}

\begin{abstract}
  We report on tests of the shifted potential (SP), gradient shifted
  force (GSF), and Taylor shifted force (TSF) real-space methods for
  multipole interactions developed in the first paper in this series,
  using the multipolar Ewald sum as a reference method. The tests were
  carried out in a variety of condensed-phase environments designed to
  test up to quadrupole-quadrupole interactions.  Comparisons of the
  energy differences between configurations, molecular forces, and
  torques were used to analyze how well the real-space models perform
  relative to the more computationally expensive Ewald treatment.  We
  have also investigated the energy conservation, structural, and
  dynamical properties of the new methods in molecular dynamics
  simulations. The SP method shows excellent agreement with
  configurational energy differences, forces, and torques, and would
  be suitable for use in Monte Carlo calculations.  Of the two new
  shifted-force methods, the GSF approach shows the best agreement
  with Ewald-derived energies, forces, and torques and also exhibits
  energy conservation properties that make it an excellent choice for
  efficient computation of electrostatic interactions in molecular
  dynamics simulations.  Both SP and GSF are able to reproduce
  structural and dyanamical properties in the liquid models with
  excellent fidelity.
\end{abstract}

%\pacs{Valid PACS appear here}% PACS, the Physics and Astronomy
                             % Classification Scheme.
%\keywords{Electrostatics, Multipoles, Real-space}

\maketitle

\section{\label{sec:intro}Introduction}
Computing the interactions between electrostatic sites is one of the
most expensive aspects of molecular simulations. There have been
significant efforts to develop practical, efficient and convergent
methods for handling these interactions. Ewald's method is perhaps the
best known and most accurate method for evaluating energies, forces,
and torques in explicitly-periodic simulation cells. In this approach,
the conditionally convergent electrostatic energy is converted into
two absolutely convergent contributions, one which is carried out in
real space with a cutoff radius, and one in reciprocal
space.\cite{Ewald21,deLeeuw80,Smith81,Allen87} 

When carried out as originally formulated, the reciprocal-space
portion of the Ewald sum exhibits relatively poor computational
scaling, making it prohibitive for large systems. By utilizing a
particle mesh and three dimensional fast Fourier transforms (FFT), the
particle-mesh Ewald (PME), particle-particle particle-mesh Ewald
(P\textsuperscript{3}ME), and smooth particle mesh Ewald (SPME)
methods can decrease the computational cost from $O(N^2)$ down to $O(N
\log
N)$.\cite{Takada93,Gunsteren94,Gunsteren95,Darden:1993pd,Essmann:1995pb}

Because of the artificial periodicity required for the Ewald sum,
interfacial molecular systems such as membranes and liquid-vapor
interfaces require modifications to the method.  Parry's extension of
the three dimensional Ewald sum is appropriate for slab
geometries.\cite{Parry:1975if} Modified Ewald methods that were
developed to handle two-dimensional (2-D) electrostatic
interactions.\cite{Parry:1975if,Parry:1976fq,Clarke77,Perram79,Rhee:1989kl}
These methods were originally quite computationally
expensive.\cite{Spohr97,Yeh99} There have been several successful
efforts that reduced the computational cost of 2-D lattice summations,
bringing them more in line with the scaling for the full 3-D
treatments.\cite{Yeh99,Kawata01,Arnold02,deJoannis02,Brodka04} The
inherent periodicity required by the Ewald method can also be
problematic in a number of protein/solvent and ionic solution
environments.\cite{Roberts94,Roberts95,Luty96,Hunenberger99a,Hunenberger99b,Weber00,Fennell:2006lq}

\subsection{Real-space methods}
Wolf \textit{et al.}\cite{Wolf:1999dn} proposed a real space $O(N)$
method for calculating electrostatic interactions between point
charges. They argued that the effective Coulomb interaction in most
condensed phase systems is effectively short
ranged.\cite{Wolf92,Wolf95} For an ordered lattice (e.g., when
computing the Madelung constant of an ionic solid), the material can
be considered as a set of ions interacting with neutral dipolar or
quadrupolar ``molecules'' giving an effective distance dependence for
the electrostatic interactions of $r^{-5}$ (see figure
\ref{fig:schematic}). If one views the \ce{NaCl} crystal as a simple
cubic (SC) structure with an octupolar \ce{(NaCl)4} basis, the
electrostatic energy per ion converges more rapidly to the Madelung
energy than the dipolar approximation.\cite{Wolf92} To find the
correct Madelung constant, Lacman suggested that the NaCl structure
could be constructed in a way that the finite crystal terminates with
complete \ce{(NaCl)4} molecules.\cite{Lacman65} The central ion sees
what is effectively a set of octupoles at large distances. These facts
suggest that the Madelung constants are relatively short ranged for
perfect ionic crystals.\cite{Wolf:1999dn} For this reason, careful
application of Wolf's method can provide accurate estimates of
Madelung constants using relatively short cutoff radii.

Direct truncation of interactions at a cutoff radius creates numerical
errors.  Wolf \textit{et al.} suggest that truncation errors are due
to net charge remaining inside the cutoff sphere.\cite{Wolf:1999dn} To
neutralize this charge they proposed placing an image charge on the
surface of the cutoff sphere for every real charge inside the cutoff.
These charges are present for the evaluation of both the pair
interaction energy and the force, although the force expression
maintains a discontinuity at the cutoff sphere.  In the original Wolf
formulation, the total energy for the charge and image were not equal
to the integral of the force expression, and as a result, the total
energy would not be conserved in molecular dynamics (MD)
simulations.\cite{Zahn:2002hc} Zahn \textit{et al.}, and Fennel and
Gezelter later proposed shifted force variants of the Wolf method with
commensurate force and energy expressions that do not exhibit this
problem.\cite{Zahn:2002hc,Fennell:2006lq} Related real-space methods
were also proposed by Chen \textit{et
  al.}\cite{Chen:2004du,Chen:2006ii,Denesyuk:2008ez,Rodgers:2006nw}
and by Wu and Brooks.\cite{Wu:044107} Recently, Fukuda has successfuly
used additional neutralization of higher order moments for systems of
point charges.\cite{Fukuda:2013sf}

\begin{figure}
  \centering
  \includegraphics[width=\linewidth]{schematic.eps}
  \caption{Top: Ionic systems exhibit local clustering of dissimilar
    charges (in the smaller grey circle), so interactions are
    effectively charge-multipole at longer distances.  With hard
    cutoffs, motion of individual charges in and out of the cutoff
    sphere can break the effective multipolar ordering.  Bottom:
    dipolar crystals and fluids have a similar effective
    \textit{quadrupolar} ordering (in the smaller grey circles), and
    orientational averaging helps to reduce the effective range of the
    interactions in the fluid.  Placement of reversed image multipoles
    on the surface of the cutoff sphere recovers the effective
    higher-order multipole behavior. \label{fig:schematic}}
\end{figure}

One can make a similar effective range argument for crystals of point
\textit{multipoles}. The Luttinger and Tisza treatment of energy
constants for dipolar lattices utilizes 24 basis vectors that contain
dipoles at the eight corners of a unit cube.\cite{LT} Only three of
these basis vectors, $X_1, Y_1, \mathrm{~and~} Z_1,$ retain net dipole
moments, while the rest have zero net dipole and retain contributions
only from higher order multipoles.  The lowest-energy crystalline
structures are built out of basis vectors that have only residual
quadrupolar moments (e.g. the $Z_5$ array). In these low energy
structures, the effective interaction between a dipole at the center
of a crystal and a group of eight dipoles farther away is
significantly shorter ranged than the $r^{-3}$ that one would expect
for raw dipole-dipole interactions.  Only in crystals which retain a
bulk dipole moment (e.g. ferroelectrics) does the analogy with the
ionic crystal break down -- ferroelectric dipolar crystals can exist,
while ionic crystals with net charge in each unit cell would be
unstable.

In ionic crystals, real-space truncation can break the effective
multipolar arrangements (see Fig. \ref{fig:schematic}), causing
significant swings in the electrostatic energy as individual ions move
back and forth across the boundary.  This is why the image charges are
necessary for the Wolf sum to exhibit rapid convergence.  Similarly,
the real-space truncation of point multipole interactions breaks
higher order multipole arrangements, and image multipoles are required
for real-space treatments of electrostatic energies.

The shorter effective range of electrostatic interactions is not
limited to perfect crystals, but can also apply in disordered fluids.
Even at elevated temperatures, there is local charge balance in an
ionic liquid, where each positive ion has surroundings dominated by
negative ions and vice versa.  The reversed-charge images on the
cutoff sphere that are integral to the Wolf and DSF approaches retain
the effective multipolar interactions as the charges traverse the
cutoff boundary.

In multipolar fluids (see Fig. \ref{fig:schematic}) there is
significant orientational averaging that additionally reduces the
effect of long-range multipolar interactions.  The image multipoles
that are introduced in the TSF, GSF, and SP methods mimic this effect
and reduce the effective range of the multipolar interactions as
interacting molecules traverse each other's cutoff boundaries.

Forces and torques acting on atomic sites are fundamental in driving
dynamics in molecular simulations, and the damped shifted force (DSF)
energy kernel provides consistent energies and forces on charged atoms
within the cutoff sphere. Both the energy and the force go smoothly to
zero as an atom aproaches the cutoff radius. The comparisons of the
accuracy these quantities between the DSF kernel and SPME was
surprisingly good.\cite{Fennell:2006lq} As a result, the DSF method
has seen increasing use in molecular systems with relatively uniform
charge
densities.\cite{English08,Kannam:2012rr,Space12,Lawrence13,Acevedo13,Shi:2013ij,Vergne13}

\subsection{The damping function}
The damping function has been discussed in detail in the first paper
of this series.\cite{PaperI} The $1/r$ Coulombic kernel for the
interactions between point charges can be replaced by the
complementary error function $\mathrm{erfc}(\alpha r)/r$ to accelerate
convergence, where $\alpha$ is a damping parameter with units of
inverse distance.  Altering the value of $\alpha$ is equivalent to
changing the width of Gaussian charge distributions that replace each
point charge, as Coulomb integrals with Gaussian charge distributions
produce complementary error functions when truncated at a finite
distance.

With moderate damping coefficients, $\alpha \sim 0.2$, the DSF method
produced very good agreement with SPME for interaction energies,
forces and torques for charge-charge
interactions.\cite{Fennell:2006lq}

\subsection{Point multipoles in molecular modeling}
Coarse-graining approaches which treat entire molecular subsystems as
a single rigid body are now widely used. A common feature of many
coarse-graining approaches is simplification of the electrostatic
interactions between bodies so that fewer site-site interactions are
required to compute configurational
energies.\cite{Ren06,Essex10,Essex11}

Additionally, because electrons in a molecule are not localized at
specific points, the assignment of partial charges to atomic centers
is always an approximation.  For increased accuracy, atomic sites can
also be assigned point multipoles and polarizabilities.  Recently,
water has been modeled with point multipoles up to octupolar order
using the soft sticky dipole-quadrupole-octupole (SSDQO)
model.\cite{Ichiye10_1,Ichiye10_2,Ichiye10_3} Atom-centered point
multipoles up to quadrupolar order have also been coupled with point
polarizabilities in the high-quality AMOEBA and iAMOEBA water
models.\cite{Ren:2003uq,Ren:2004kx,Ponder:2010vl,Wang:2013fk} However,
truncating point multipoles without smoothing the forces and torques
can create energy conservation issues in molecular dynamics
simulations.

In this paper we test a set of real-space methods that were developed
for point multipolar interactions.  These methods extend the damped
shifted force (DSF) and Wolf methods originally developed for
charge-charge interactions and generalize them for higher order
multipoles.  The detailed mathematical development of these methods
has been presented in the first paper in this series, while this work
covers the testing of energies, forces, torques, and energy
conservation properties of the methods in realistic simulation
environments.  In all cases, the methods are compared with the
reference method, a full multipolar Ewald treatment.\cite{Smith82,Smith98}


\section{\label{sec:method}Review of Methods}
Any real-space electrostatic method that is suitable for MD
simulations should have the electrostatic energy, forces and torques
between two sites go smoothly to zero as the distance between the
sites, $r_{ab}$ approaches the cutoff radius, $r_c$.  Requiring
this continuity at the cutoff is essential for energy conservation in
MD simulations.  The mathematical details of the shifted potential
(SP), gradient-shifted-force (GSF) and Taylor shifted-force (TSF)
methods have been discussed in detail in the previous paper in this
series.\cite{PaperI} Here we briefly review the new methods and
describe their essential features.

\subsection{Taylor-shifted force (TSF)}

The electrostatic potential energy between point multipoles can be
expressed as the product of two multipole operators and a Coulombic
kernel,
\begin{equation}
U_{ab}(r)= M_{a} M_{b} \frac{1}{r}  \label{kernel}.
\end{equation}
where the multipole operator for site $a$, $M_{a}$, is
expressed in terms of the point charge, $C_{a}$, dipole, ${\bf D}_{a}$, and quadrupole, $\mathsf{Q}_{a}$, for object
$a$, etc.

The TSF potential for any multipole-multipole interaction can be
written
\begin{equation}
U^{\text{TSF}}= (\text{prefactor}) (\text{derivatives}) f_n(r)
\label{generic}
\end{equation}
where $f_n(r)$ is a shifted kernel that is appropriate for the order
of the interaction (see Ref. \onlinecite{PaperI}), with $n=0$ for
charge-charge, $n=1$ for charge-dipole, $n=2$ for charge-quadrupole
and dipole-dipole, $n=3$ for dipole-quadrupole, and $n=4$ for
quadrupole-quadrupole.  To ensure smooth convergence of the energy,
force, and torques, a Taylor expansion with $n$ terms must be
performed at cutoff radius ($r_c$) to obtain $f_n(r)$.

For multipole-multipole interactions, following this procedure results
in separate radial functions for each of the distinct orientational
contributions to the potential, and ensures that the forces and
torques from each of these contributions will vanish at the cutoff
radius.  For example, the direct dipole dot product
($\mathbf{D}_{a}
\cdot \mathbf{D}_{b}$) is treated differently than the dipole-distance
dot products:
\begin{equation}
U_{D_{a}D_{b}}(r)= -\frac{1}{4\pi \epsilon_0} \left[ \left(
  \mathbf{D}_{a} \cdot
\mathbf{D}_{b} \right) v_{21}(r) +
\left( \mathbf{D}_{a} \cdot \hat{\mathbf{r}} \right)
\left( \mathbf{D}_{b} \cdot \hat{\mathbf{r}} \right) v_{22}(r) \right]
\end{equation}

For the Taylor shifted (TSF) method with the undamped kernel,
$v_{21}(r) = -\frac{1}{r^3} + \frac{3 r}{r_c^4} - \frac{8}{r_c^3} +
\frac{6}{r r_c^2}$ and $v_{22}(r) = \frac{3}{r^3} + \frac{3 r}{r_c^4}
- \frac{6}{r r_c^2}$.  In these functions, one can easily see the
connection to unmodified electrostatics as well as the smooth
transition to zero in both these functions as $r\rightarrow r_c$.  The
electrostatic forces and torques acting on the central multipole due
to another site within the cutoff sphere are derived from
Eq.~\ref{generic}, accounting for the appropriate number of
derivatives. Complete energy, force, and torque expressions are
presented in the first paper in this series (Reference
\onlinecite{PaperI}).

\subsection{Gradient-shifted force (GSF)}

A second (and conceptually simpler) method involves shifting the
gradient of the raw Coulomb potential for each particular multipole
order.  For example, the raw dipole-dipole potential energy may be
shifted smoothly by finding the gradient for two interacting dipoles
which have been projected onto the surface of the cutoff sphere
without changing their relative orientation,
\begin{equation}
U_{D_{a}D_{b}}(r)  = U_{D_{a}D_{b}}(r) -
U_{D_{a}D_{b}}(r_c)
   - (r_{ab}-r_c) ~~~\hat{\mathbf{r}}_{ab} \cdot
  \nabla U_{D_{a}D_{b}}(r_c).
\end{equation}
Here the lab-frame orientations of the two dipoles, $\mathbf{D}_{a}$ and $\mathbf{D}_{b}$, are retained at the cutoff distance
(although the signs are reversed for the dipole that has been
projected onto the cutoff sphere).  In many ways, this simpler
approach is closer in spirit to the original shifted force method, in
that it projects a neutralizing multipole (and the resulting forces
from this multipole) onto a cutoff sphere. The resulting functional
forms for the potentials, forces, and torques turn out to be quite
similar in form to the Taylor-shifted approach, although the radial
contributions are significantly less perturbed by the gradient-shifted
approach than they are in the Taylor-shifted method.

For the gradient shifted (GSF) method with the undamped kernel,
$v_{21}(r) = -\frac{1}{r^3} - \frac{3 r}{r_c^4} + \frac{4}{r_c^3}$ and
$v_{22}(r) = \frac{3}{r^3} + \frac{9 r}{r_c^4} - \frac{12}{r_c^3}$.
Again, these functions go smoothly to zero as $r\rightarrow r_c$, and
because the Taylor expansion retains only one term, they are
significantly less perturbed than the TSF functions.

In general, the gradient shifted potential between a central multipole
and any multipolar site inside the cutoff radius is given by,
\begin{equation}
U^{\text{GSF}} = \sum \left[ U(\mathbf{r}, \mathsf{A}, \mathsf{B}) -
U(r_c \hat{\mathbf{r}},\mathsf{A}, \mathsf{B}) - (r-r_c)
\hat{\mathbf{r}} \cdot \nabla U(r_c \hat{\mathbf{r}},\mathsf{A}, \mathsf{B}) \right]
\label{generic2}
\end{equation}
where the sum describes a separate force-shifting that is applied to
each orientational contribution to the energy.  In this expression,
$\hat{\mathbf{r}}$ is the unit vector connecting the two multipoles
($a$ and $b$) in space, and $\mathsf{A}$ and $\mathsf{B}$
represent the orientations the multipoles.

The third term converges more rapidly than the first two terms as a
function of radius, hence the contribution of the third term is very
small for large cutoff radii.  The force and torque derived from
Eq. \ref{generic2} are consistent with the energy expression and
approach zero as $r \rightarrow r_c$.  Both the GSF and TSF methods
can be considered generalizations of the original DSF method for
higher order multipole interactions. GSF and TSF are also identical up
to the charge-dipole interaction but generate different expressions in
the energy, force and torque for higher order multipole-multipole
interactions. Complete energy, force, and torque expressions for the
GSF potential are presented in the first paper in this series
(Reference~\onlinecite{PaperI}).


\subsection{Shifted potential (SP) }
A discontinuous truncation of the electrostatic potential at the
cutoff sphere introduces a severe artifact (oscillation in the
electrostatic energy) even for molecules with the higher-order
multipoles.\cite{PaperI} We have also formulated an extension of the
Wolf approach for point multipoles by simply projecting the image
multipole onto the surface of the cutoff sphere, and including the
interactions with the central multipole and the image. This
effectively shifts the total potential to zero at the cutoff radius,
\begin{equation}
U^{\text{SP}} = \sum \left[ U(\mathbf{r}, \mathsf{A}, \mathsf{B}) -
U(r_c \hat{\mathbf{r}},\mathsf{A}, \mathsf{B}) \right]
\label{eq:SP}
\end{equation}          
where the sum describes separate potential shifting that is done for
each orientational contribution to the energy (e.g. the direct dipole
product contribution is shifted {\it separately} from the
dipole-distance terms in dipole-dipole interactions).  Note that this
is not a simple shifting of the total potential at $r_c$. Each radial
contribution is shifted separately.  One consequence of this is that
multipoles that reorient after leaving the cutoff sphere can re-enter
the cutoff sphere without perturbing the total energy.

For the shifted potential (SP) method with the undamped kernel,
$v_{21}(r) = -\frac{1}{r^3} + \frac{1}{r_c^3}$ and $v_{22}(r) =
\frac{3}{r^3} - \frac{3}{r_c^3}$.  The potential energy between a
central multipole and other multipolar sites goes smoothly to zero as
$r \rightarrow r_c$.  However, the force and torque obtained from the
shifted potential (SP) are discontinuous at $r_c$.  MD simulations
will still experience energy drift while operating under the SP
potential, but it may be suitable for Monte Carlo approaches where the
configurational energy differences are the primary quantity of
interest.

\subsection{The Self Term}
In the TSF, GSF, and SP methods, a self-interaction is retained for
the central multipole interacting with its own image on the surface of
the cutoff sphere.  This self interaction is nearly identical with the
self-terms that arise in the Ewald sum for multipoles.  Complete
expressions for the self terms are presented in the first paper in
this series (Reference \onlinecite{PaperI}). 


\section{\label{sec:methodology}Methodology}

To understand how the real-space multipole methods behave in computer
simulations, it is vital to test against established methods for
computing electrostatic interactions in periodic systems, and to
evaluate the size and sources of any errors that arise from the
real-space cutoffs.  In the first paper of this series, we compared
the dipolar and quadrupolar energy expressions against analytic
expressions for ordered dipolar and quadrupolar
arrays.\cite{Sauer,LT,Nagai01081960,Nagai01091963} In this work, we
used the multipolar Ewald sum as a reference method for comparing
energies, forces, and torques for molecular models that mimic
disordered and ordered condensed-phase systems.  The parameters used
in the test cases are given in table~\ref{tab:pars}. 

\begin{table}
\caption{The parameters used in the systems used to evaluate the new
  real-space methods.  The most comprehensive test was a liquid
  composed of 2000 SSDQ molecules with 48 dissolved ions (24 \ce{Na+} and 24 \ce{Cl-}
  ions).  This test excercises all orders of the multipolar
  interactions developed in the first paper.\label{tab:pars}}
\begin{tabularx}{\textwidth}{r|cc|YYccc|Yccc} \hline
             & \multicolumn{2}{c|}{LJ parameters} &
             \multicolumn{5}{c|}{Electrostatic moments} & & & & \\
 Test system & $\sigma$& $\epsilon$ & $C$ & $D$  &
 $Q_{xx}$ & $Q_{yy}$ & $Q_{zz}$ & mass  & $I_{xx}$ & $I_{yy}$ &
 $I_{zz}$ \\ \cline{6-8}\cline{10-12}
 & (\AA) & (kcal/mol) & (e) & (debye) & \multicolumn{3}{c|}{(debye \AA)} & (amu) & \multicolumn{3}{c}{(amu
 \AA\textsuperscript{2})} \\ \hline
    Soft Dipolar fluid & 3.051 & 0.152 &  & 2.35 & & & & 18.0153 & 1.77&0.6145& 1.155 \\
    Soft Dipolar solid & 2.837 & 1.0   &  & 2.35 & & & & $10^4$  & 17.6 &17.6 & 0 \\
Soft Quadrupolar fluid & 3.051 & 0.152 &  &  & -1&-1&-2.5 & 18.0153 & 1.77&0.6145&1.155  \\
Soft Quadrupolar solid & 2.837 & 1.0   &  &  & -1&-1&-2.5 & $10^4$  & 17.6&17.6&0 \\
      SSDQ water  & 3.051 & 0.152 &  & 2.35 & -1.35&0&-0.68 & 18.0153 & 1.77&0.6145&1.155 \\
              \ce{Na+} & 2.579 & 0.118 & +1& & & & & 22.99 & & &\\
              \ce{Cl-} & 4.445 & 0.1   & -1& & & & & 35.4527& & & \\ \hline
\end{tabularx}
\end{table}
The systems consist of pure multipolar solids (both dipole and
quadrupole), pure multipolar liquids (both dipole and quadrupole), a
fluid composed of sites containing both dipoles and quadrupoles
simultaneously, and a final test case that includes ions with point
charges in addition to the multipolar fluid.  The solid-phase
parameters were chosen so that the systems can explore some
orientational freedom for the multipolar sites, while maintaining
relatively strict translational order.  The SSDQ model used here is
not a particularly accurate water model, but it does test
dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole
interactions at roughly the same magnitudes. The last test case, SSDQ
water with dissolved ions, exercises \textit{all} levels of the
multipole-multipole interactions we have derived so far and represents
the most complete test of the new methods.

In the following section, we present results for the total
electrostatic energy, as well as the electrostatic contributions to
the force and torque on each molecule.  These quantities have been
computed using the SP, TSF, and GSF methods, as well as a hard cutoff,
and have been compared with the values obtained from the multipolar
Ewald sum.  In Monte Carlo (MC) simulations, the energy differences
between two configurations is the primary quantity that governs how
the simulation proceeds. These differences are the most important
indicators of the reliability of a method even if the absolute
energies are not exact.  For each of the multipolar systems listed
above, we have compared the change in electrostatic potential energy
($\Delta E$) between 250 statistically-independent configurations.  In
molecular dynamics (MD) simulations, the forces and torques govern the
behavior of the simulation, so we also compute the electrostatic
contributions to the forces and torques.

\subsection{Implementation}
The real-space methods developed in the first paper in this series
have been implemented in our group's open source molecular simulation
program, OpenMD,\cite{Meineke05,openmd} which was used for all calculations in
this work.  The complementary error function can be a relatively slow
function on some processors, so all of the radial functions are
precomputed on a fine grid and are spline-interpolated to provide
values when required.  

Using the same simulation code, we compare to a multipolar Ewald sum
with a reciprocal space cutoff, $k_\mathrm{max} = 7$.  Our version of
the Ewald sum is a re-implementation of the algorithm originally
proposed by Smith that does not use the particle mesh or smoothing
approximations.\cite{Smith82,Smith98} This implementation was tested
extensively against the analytic energy constants for the multipolar
lattices that are discussed in reference \onlinecite{PaperI}.  In all
cases discussed below, the quantities being compared are the
electrostatic contributions to energies, force, and torques.  All
other contributions to these quantities (i.e. from Lennard-Jones
interactions) are removed prior to the comparisons.

The convergence parameter ($\alpha$) also plays a role in the balance
of the real-space and reciprocal-space portions of the Ewald
calculation.  Typical molecular mechanics packages set this to a value
that depends on the cutoff radius and a tolerance (typically less than
$1 \times 10^{-4}$ kcal/mol).  Smaller tolerances are typically
associated with increasing accuracy at the expense of computational
time spent on the reciprocal-space portion of the
summation.\cite{Perram88,Essmann:1995pb} A default tolerance of $1 \times
10^{-8}$ kcal/mol was used in all Ewald calculations, resulting in
Ewald coefficient 0.3119 \AA$^{-1}$ for a cutoff radius of 12 \AA.

The real-space models have self-interactions that provide
contributions to the energies only.  Although the self interaction is
a rapid calculation, we note that in systems with fluctuating charges
or point polarizabilities, the self-term is not static and must be
recomputed at each time step.

\subsection{Model systems}
To sample independent configurations of the multipolar crystals, body
centered cubic (bcc) crystals, which exhibit the minimum energy
structures for point dipoles, were generated using 3,456 molecules.
The multipoles were translationally locked in their respective crystal
sites for equilibration at a relatively low temperature (50K) so that
dipoles or quadrupoles could freely explore all accessible
orientations.  The translational constraints were then removed, the
systems were re-equilibrated, and the crystals were simulated for an
additional 10 ps in the microcanonical (NVE) ensemble with an average
temperature of 50 K.  The balance between moments of inertia and
particle mass were chosen to allow orientational sampling without
significant translational motion.  Configurations were sampled at
equal time intervals in order to compare configurational energy
differences.  The crystals were simulated far from the melting point
in order to avoid translational deformation away of the ideal lattice
geometry.

For dipolar, quadrupolar, and mixed-multipole \textit{liquid}
simulations, each system was created with 2,048 randomly-oriented
molecules.  These were equilibrated at a temperature of 300K for 1 ns.
Each system was then simulated for 1 ns in the microcanonical (NVE)
ensemble.  We collected 250 different configurations at equal time
intervals. For the liquid system that included ionic species, we
converted 48 randomly-distributed molecules into 24 \ce{Na+} and 24
\ce{Cl-} ions and re-equilibrated. After equilibration, the system was
run under the same conditions for 1 ns. A total of 250 configurations
were collected. In the following comparisons of energies, forces, and
torques, the Lennard-Jones potentials were turned off and only the
purely electrostatic quantities were compared with the same values
obtained via the Ewald sum.

\subsection{Accuracy of Energy Differences, Forces and Torques}
The pairwise summation techniques (outlined above) were evaluated for
use in MC simulations by studying the energy differences between
different configurations.  We took the Ewald-computed energy
difference between two conformations to be the correct behavior. An
ideal performance by one of the new methods would reproduce these
energy differences exactly. The configurational energies being used
here contain only contributions from electrostatic interactions.
Lennard-Jones interactions were omitted from the comparison as they
should be identical for all methods.

Since none of the real-space methods provide exact energy differences,
we used least square regressions analysis for the six different
molecular systems to compare $\Delta E$ from Hard, SP, GSF, and TSF
with the multipolar Ewald reference method.  Unitary results for both
the correlation (slope) and correlation coefficient for these
regressions indicate perfect agreement between the real-space method
and the multipolar Ewald sum.

Molecular systems were run long enough to explore independent
configurations and 250 configurations were recorded for comparison.
Each system provided 31,125 energy differences for a total of 186,750
data points.  Similarly, the magnitudes of the forces and torques have
also been compared using least squares regression analysis. In the
forces and torques comparison, the magnitudes of the forces acting in
each molecule for each configuration were evaluated. For example, our
dipolar liquid simulation contains 2048 molecules and there are 250
different configurations for each system resulting in 3,072,000 data
points for comparison of forces and torques.

\subsection{Analysis of vector quantities}
Getting the magnitudes of the force and torque vectors correct is only
part of the issue for carrying out accurate molecular dynamics
simulations.  Because the real space methods reweight the different
orientational contributions to the energies, it is also important to
understand how the methods impact the \textit{directionality} of the
force and torque vectors. Fisher developed a probablity density
function to analyse directional data sets,
\begin{equation}
p_f(\theta) = \frac{\kappa}{2 \sinh\kappa}\sin\theta e^{\kappa \cos\theta}
\label{eq:pdf}
\end{equation}
where $\kappa$ measures directional dispersion of the data around the
mean direction.\cite{fisher53} This quantity $(\kappa)$ can be
estimated as a reciprocal of the circular variance.\cite{Allen91} To
quantify the directional error, forces obtained from the Ewald sum
were taken as the mean (or correct) direction and the angle between
the forces obtained via the Ewald sum and the real-space methods were
evaluated,
\begin{equation}
\cos\theta_i =  \frac{\vec{f}_i^\mathrm{~Ewald} \cdot
  \vec{f}_i^\mathrm{~GSF}}{\left|\vec{f}_i^\mathrm{~Ewald}\right| \left|\vec{f}_i^\mathrm{~GSF}\right|}
\end{equation}
The total angular displacement of the vectors was calculated as,
\begin{equation}
R = \sqrt{\left(\sum\limits_{i=1}^N \cos\theta_i\right)^2 + \left(\sum\limits_{i=1}^N \sin\theta_i\right)^2}
\label{eq:displacement}
\end{equation} 
where $N$ is number of force vectors.  The circular variance is
defined as
\begin{equation}
\mathrm{Var}(\theta) \approx 1/\kappa = 1 - R/N
\end{equation}
The circular variance takes on values between from 0 to 1, with 0
indicating a perfect directional match between the Ewald force vectors
and the real-space forces. Lower values of $\mathrm{Var}(\theta)$
correspond to higher values of $\kappa$, which indicates tighter
clustering of the real-space force vectors around the Ewald forces.

A similar analysis was carried out for the electrostatic contribution
to the molecular torques as well as forces.  

\subsection{Energy conservation}
To test conservation the energy for the methods, the mixed molecular
system of 2000 SSDQ water molecules with 24 \ce{Na+} and 24 \ce{Cl-}
ions was run for 1 ns in the microcanonical ensemble at an average
temperature of 300K.  Each of the different electrostatic methods
(Ewald, Hard, SP, GSF, and TSF) was tested for a range of different
damping values. The molecular system was started with same initial
positions and velocities for all cutoff methods. The energy drift
($\delta E_1$) and standard deviation of the energy about the slope
($\delta E_0$) were evaluated from the total energy of the system as a
function of time.  Although both measures are valuable at
investigating new methods for molecular dynamics, a useful interaction
model must allow for long simulation times with minimal energy drift.

\section{\label{sec:result}RESULTS}
\subsection{Configurational energy differences}
 
\begin{figure}
  \centering
  \includegraphics[width=0.85\linewidth]{energyPlot_slopeCorrelation_combined.eps}
  \caption{Statistical analysis of the quality of configurational
    energy differences for the real-space electrostatic methods
    compared with the reference Ewald sum.  Results with a value equal
    to 1 (dashed line) indicate $\Delta E$ values indistinguishable
    from those obtained using the multipolar Ewald sum.  Different
    values of the cutoff radius are indicated with different symbols
    (9\AA\ = circles, 12\AA\ = squares, and 15\AA\ = inverted
    triangles).\label{fig:slopeCorr_energy}}
\end{figure} 

The combined correlation coefficient and slope for all six systems is
shown in Figure ~\ref{fig:slopeCorr_energy}.  Most of the methods
reproduce the Ewald configurational energy differences with remarkable
fidelity.  Undamped hard cutoffs introduce a significant amount of
random scatter in the energy differences which is apparent in the
reduced value of the correlation coefficient for this method.  This
can be easily understood as configurations which exhibit small
traversals of a few dipoles or quadrupoles out of the cutoff sphere
will see large energy jumps when hard cutoffs are used.  The
orientations of the multipoles (particularly in the ordered crystals)
mean that these energy jumps can go in either direction, producing a
significant amount of random scatter, but no systematic error.

The TSF method produces energy differences that are highly correlated
with the Ewald results, but it also introduces a significant
systematic bias in the values of the energies, particularly for
smaller cutoff values. The TSF method alters the distance dependence
of different orientational contributions to the energy in a
non-uniform way, so the size of the cutoff sphere can have a large
effect, particularly for the crystalline systems.

Both the SP and GSF methods appear to reproduce the Ewald results with
excellent fidelity, particularly for moderate damping ($\alpha =
0.1-0.2$\AA$^{-1}$) and with a commonly-used cutoff value ($r_c =
12$\AA).  With the exception of the undamped hard cutoff, and the TSF
method with short cutoffs, all of the methods would be appropriate for
use in Monte Carlo simulations.
 
\subsection{Magnitude of the force and torque vectors}

The comparisons of the magnitudes of the forces and torques for the
data accumulated from all six systems are shown in Figures
~\ref{fig:slopeCorr_force} and \ref{fig:slopeCorr_torque}. The
correlation and slope for the forces agree well with the Ewald sum
even for the hard cutoffs.

For systems of molecules with only multipolar interactions, the pair
energy contributions are quite short ranged.  Moreover, the force
decays more rapidly than the electrostatic energy, hence the hard
cutoff method can also produce reasonable agreement for this quantity.
Although the pure cutoff gives reasonably good electrostatic forces
for pairs of molecules included within each other's cutoff spheres,
the discontinuity in the force at the cutoff radius can potentially
cause energy conservation problems as molecules enter and leave the
cutoff spheres.  This is discussed in detail in section
\ref{sec:conservation}.

The two shifted-force methods (GSF and TSF) exhibit a small amount of
systematic variation and scatter compared with the Ewald forces.  The
shifted-force models intentionally perturb the forces between pairs of
molecules inside each other's cutoff spheres in order to correct the
energy conservation issues, and this perturbation is evident in the
statistics accumulated for the molecular forces.  The GSF
perturbations are minimal, particularly for moderate damping and
commonly-used cutoff values ($r_c = 12$\AA).  The TSF method shows
reasonable agreement in the correlation coefficient but again the
systematic error in the forces is concerning if replication of Ewald
forces is desired.

It is important to note that the forces and torques from the SP and
the Hard cutoffs are not identical. The SP method shifts each
orientational contribution separately (e.g. the dipole-dipole dot
product is shifted by a different function than the dipole-distance
products), while the hard cutoff contains no orientation-dependent
shifting.  The forces and torques for these methods therefore diverge
for multipoles even though the forces for point charges are identical.

\begin{figure}
  \centering
  \includegraphics[width=0.6\linewidth]{forcePlot_slopeCorrelation_combined.eps} 
  \caption{Statistical analysis of the quality of the force vector
    magnitudes for the real-space electrostatic methods compared with
    the reference Ewald sum. Results with a value equal to 1 (dashed
    line) indicate force magnitude values indistinguishable from those
    obtained using the multipolar Ewald sum.  Different values of the
    cutoff radius are indicated with different symbols (9\AA\ =
    circles, 12\AA\ = squares, and 15\AA\ = inverted
    triangles).\label{fig:slopeCorr_force}}
\end{figure} 


\begin{figure}
  \centering
  \includegraphics[width=0.6\linewidth]{torquePlot_slopeCorrelation_combined.eps}
  \caption{Statistical analysis of the quality of the torque vector
    magnitudes for the real-space electrostatic methods compared with
    the reference Ewald sum. Results with a value equal to 1 (dashed
    line) indicate force magnitude values indistinguishable from those
    obtained using the multipolar Ewald sum.  Different values of the
    cutoff radius are indicated with different symbols (9\AA\ =
    circles, 12\AA\ = squares, and 15\AA\ = inverted
    triangles).\label{fig:slopeCorr_torque}}
\end{figure}

The torques (Fig. \ref{fig:slopeCorr_torque}) appear to be
significantly influenced by the choice of real-space method.  The
torque expressions have the same distance dependence as the energies,
which are naturally longer-ranged expressions than the inter-site
forces.  Torques are also quite sensitive to orientations of
neighboring molecules, even those that are near the cutoff distance.

The results shows that the torque from the hard cutoff method
reproduces the torques in quite good agreement with the Ewald sum.
The other real-space methods can cause some deviations, but excellent
agreement with the Ewald sum torques is recovered at moderate values
of the damping coefficient ($\alpha = 0.1-0.2$\AA$^{-1}$) and cutoff
radius ($r_c \ge 12$\AA).  The TSF method exhibits only fair agreement
in the slope when compared with the Ewald torques even for larger
cutoff radii.  It appears that the severity of the perturbations in
the TSF method are most in evidence for the torques.

\subsection{Directionality of the force and torque vectors}   

The accurate evaluation of force and torque directions is just as
important for molecular dynamics simulations as the magnitudes of
these quantities. Force and torque vectors for all six systems were
analyzed using Fisher statistics, and the quality of the vector
directionality is shown in terms of circular variance
($\mathrm{Var}(\theta)$) in figure
\ref{fig:slopeCorr_circularVariance}. The force and torque vectors
from the new real-space methods exhibit nearly-ideal Fisher probability
distributions (Eq.~\ref{eq:pdf}). Both the hard and SP cutoff methods
exhibit the best vectorial agreement with the Ewald sum. The force and
torque vectors from GSF method also show good agreement with the Ewald
method, which can also be systematically improved by using moderate
damping and a reasonable cutoff radius. For $\alpha = 0.2$ and $r_c =
12$\AA, we observe $\mathrm{Var}(\theta) = 0.00206$, which corresponds
to a distribution with 95\% of force vectors within $6.37^\circ$ of
the corresponding Ewald forces. The TSF method produces the poorest
agreement with the Ewald force directions.

Torques are again more perturbed than the forces by the new real-space
methods, but even here the variance is reasonably small.  For the same
method (GSF) with the same parameters ($\alpha = 0.2, r_c = 12$\AA),
the circular variance was 0.01415, corresponds to a distribution which
has 95\% of torque vectors are within $16.75^\circ$ of the Ewald
results. Again, the direction of the force and torque vectors can be
systematically improved by varying $\alpha$ and $r_c$.

\begin{figure}
  \centering
  \includegraphics[width=0.65\linewidth]{Variance_forceNtorque_modified.eps}
  \caption{The circular variance of the direction of the force and
    torque vectors obtained from the real-space methods around the
    reference Ewald vectors. A variance equal to 0 (dashed line)
    indicates direction of the force or torque vectors are
    indistinguishable from those obtained from the Ewald sum. Here
    different symbols represent different values of the cutoff radius
    (9 \AA\ = circle, 12 \AA\ = square, 15 \AA\ = inverted triangle)\label{fig:slopeCorr_circularVariance}}
\end{figure}

\subsection{Energy conservation\label{sec:conservation}}

We have tested the conservation of energy one can expect to see with
the new real-space methods using the SSDQ water model with a small
fraction of solvated ions. This is a test system which exercises all
orders of multipole-multipole interactions derived in the first paper
in this series and provides the most comprehensive test of the new
methods.  A liquid-phase system was created with 2000 water molecules
and 48 dissolved ions at a density of 0.98 g cm$^{-3}$ and a
temperature of 300K.  After equilibration in the canonical (NVT)
ensemble using a Nos\'e-Hoover thermostat, this liquid-phase system
was run for 1 ns in the microcanonical (NVE) ensemble under the Ewald,
Hard, SP, GSF, and TSF methods with a cutoff radius of 12\AA.  The
value of the damping coefficient was also varied from the undamped
case ($\alpha = 0$) to a heavily damped case ($\alpha = 0.3$
\AA$^{-1}$) for all of the real space methods.  A sample was also run
using the multipolar Ewald sum with the same real-space cutoff.

In figure~\ref{fig:energyDrift} we show the both the linear drift in
energy over time, $\delta E_1$, and the standard deviation of energy
fluctuations around this drift $\delta E_0$.  Both of the
shifted-force methods (GSF and TSF) provide excellent energy
conservation (drift less than $10^{-5}$ kcal / mol / ns / particle),
while the hard cutoff is essentially unusable for molecular dynamics.
SP provides some benefit over the hard cutoff because the energetic
jumps that happen as particles leave and enter the cutoff sphere are
somewhat reduced, but like the Wolf method for charges, the SP method
would not be as useful for molecular dynamics as either of the
shifted-force methods.

We note that for all tested values of the cutoff radius, the new
real-space methods can provide better energy conservation behavior
than the multipolar Ewald sum, even when relatively large $k$-space
cutoff values are utilized.

\begin{figure}
  \centering
  \includegraphics[width=\textwidth]{newDrift_12.eps}
  \caption{Energy conservation of the real-space methods for the SSDQ
    water/ion system. $\delta \mathrm{E}_1$ is the linear drift in
    energy over time (in kcal/mol/particle/ns) and $\delta
    \mathrm{E}_0$ is the standard deviation of energy fluctuations
    around this drift (in kcal/mol/particle).  Points that appear in
    the green region at the bottom exhibit better energy conservation
    than would be obtained using common parameters for Ewald-based
    electrostatics.\label{fig:energyDrift}}
\end{figure} 

\subsection{Reproduction of Structural \& Dynamical Features\label{sec:structure}}
The most important test of the modified interaction potentials is the
fidelity with which they can reproduce structural features and
dynamical properties in a liquid.  One commonly-utilized measure of
structural ordering is the pair distribution function, $g(r)$, which
measures local density deviations in relation to the bulk density.  In
the electrostatic approaches studied here, the short-range repulsion
from the Lennard-Jones potential is identical for the various
electrostatic methods, and since short range repulsion determines much
of the local liquid ordering, one would not expect to see many
differences in $g(r)$.  Indeed, the pair distributions are essentially
identical for all of the electrostatic methods studied (for each of
the different systems under investigation).  An example of this
agreement for the SSDQ water/ion system is shown in
Fig. \ref{fig:gofr}.

\begin{figure}
  \centering
  \includegraphics[width=\textwidth]{gofr_ssdqc.eps}
\caption{The pair distribution functions, $g(r)$, for the SSDQ
  water/ion system obtained using the different real-space methods are
  essentially identical with the result from the Ewald
  treatment.\label{fig:gofr}}
\end{figure} 

There is a minor overstructuring of the first solvation shell when
using TSF or when overdamping with any of the real-space methods.
With moderate damping, GSF and SP produce pair distributions that are
identical (within numerical noise) to their Ewald counterparts.  The
degree of overstructuring can be measured most easily using the
coordination number,
\begin{equation}
n_C = 4\pi\rho \int_{0}^{a}r^2\text{g}(r)dr,
\end{equation}
where $\rho$ is the number density of the site-site pair interactions,
$a$ and is the radial location of the minima following the first peak
in $g(r)$ ($a = 4.2$ \AA for the SSDQ water/ion system).  The
coordination number is shown as a function of the damping coefficient
for all of the real space methods in Fig.  \ref{fig:Props}.

A more demanding test of modified electrostatics is the average value
of the electrostatic energy $\langle U_\mathrm{elect} \rangle / N$
which is obtained by sampling the liquid-state configurations
experienced by a liquid evolving entirely under the influence of each
of the methods.  In fig \ref{fig:Props} we demonstrate how $\langle
U_\mathrm{elect} \rangle / N$ varies with the damping parameter,
$\alpha$, for each of the methods.

As in the crystals studied in the first paper, damping is important
for converging the mean electrostatic energy values, particularly for
the two shifted force methods (GSF and TSF).  A value of $\alpha
\approx 0.2$ \AA$^{-1}$ is sufficient to converge the SP and GSF
energies with a cutoff of 12 \AA, while shorter cutoffs require more
dramatic damping ($\alpha \approx 0.28$ \AA$^{-1}$ for $r_c = 9$ \AA).
Overdamping the real-space electrostatic methods occurs with $\alpha >
0.3$, causing the estimate of the electrostatic energy to drop below
the Ewald results.

These ``optimal'' values of the damping coefficient are slightly
larger than what were observed for DSF electrostatics for purely
point-charge systems, although the range $\alpha= 0.175 \rightarrow
0.225$ \AA$^{-1}$ for $r_c = 12$\AA\ appears to be an excellent
compromise for mixed charge/multipolar systems.

To test the fidelity of the electrostatic methods at reproducing
\textit{dynamics} in a multipolar liquid, it is also useful to look at
transport properties, particularly the diffusion constant,
\begin{equation}
D = \lim_{t \rightarrow \infty} \frac{1}{6 t} \langle \left|
  \mathbf{r}(t) -\mathbf{r}(0) \right|^2 \rangle
\label{eq:diff}
\end{equation}
which measures long-time behavior and is sensitive to the forces on
the multipoles.  The self-diffusion constants (D) were calculated from
linear fits to the long-time portion of the mean square displacement,
$\langle r^{2}(t) \rangle$.\cite{Allen87} In fig. \ref{fig:Props} we
demonstrate how the diffusion constant depends on the choice of
real-space methods and the damping coefficient.  Both the SP and GSF
methods can obtain excellent agreement with Ewald again using moderate
damping.

In addition to translational diffusion, orientational relaxation times
were calculated for comparisons with the Ewald simulations and with
experiments. These values were determined from the same 1~ns
microcanonical trajectories used for translational diffusion by
calculating the orientational time correlation function,
\begin{equation}
C_l^\gamma(t) = \left\langle P_l\left[\hat{\mathbf{A}}_\gamma(t)
                \cdot\hat{\mathbf{A}}_\gamma(0)\right]\right\rangle,
\label{eq:OrientCorr}
\end{equation}
where $P_l$ is the Legendre polynomial of order $l$ and
$\hat{\mathbf{A}}_\gamma$ is the unit vector for body axis $\gamma$.
The reference frame used for our sample dipolar systems has the
$z$-axis running along the dipoles, and for the SSDQ water model, the
$y$-axis connects the two implied hydrogen atom positions.  From the
orientation autocorrelation functions, we can obtain time constants
for rotational relaxation either by fitting an exponential function or
by integrating the entire correlation function.  In a good water
model, these decay times would be comparable to water orientational
relaxation times from nuclear magnetic resonance (NMR). The relaxation
constant obtained from $C_2^y(t)$ is normally of experimental interest
because it describes the relaxation of the principle axis connecting
the hydrogen atoms. Thus, $C_2^y(t)$ can be compared to the
intermolecular portion of the dipole-dipole relaxation from a proton
NMR signal and should provide an estimate of the NMR relaxation time
constant.\cite{Impey82}

Results for the diffusion constants and orientational relaxation times
are shown in figure \ref{fig:Props}. From this data, it is apparent
that the values for both $D$ and $\tau_2$ using the Ewald sum are
reproduced with reasonable fidelity by the GSF method.

\begin{figure}
  \caption{Comparison of the structural and dynamic properties for the
    combined multipolar liquid (SSDQ water + ions) for all of the
    real-space methods with $r_c = 12$\AA. Electrostatic energies,
    $\langle U_\mathrm{elect} \rangle / N$ (in kcal / mol),
    coordination numbers, $n_C$, diffusion constants (in cm$^2$
    s$^{-1}$), and rotational correlation times (in fs) all show
    excellent agreement with Ewald results for damping coefficients in
    the range $\alpha= 0.175 \rightarrow 0.225$
    \AA$^{-1}$. \label{fig:Props}}
  \includegraphics[width=\textwidth]{properties.eps}
\end{figure}


\section{CONCLUSION}
In the first paper in this series, we generalized the
charge-neutralized electrostatic energy originally developed by Wolf
\textit{et al.}\cite{Wolf:1999dn} to multipole-multipole interactions
up to quadrupolar order.  The SP method is essentially a
multipole-capable version of the Wolf model.  The SP method for
multipoles provides excellent agreement with Ewald-derived energies,
forces and torques, and is suitable for Monte Carlo simulations,
although the forces and torques retain discontinuities at the cutoff
distance that prevents its use in molecular dynamics.

We also developed two natural extensions of the damped shifted-force
(DSF) model originally proposed by Zahn {\it et al.} and extended by
Fennel and Gezelter.\cite{Zahn:2002hc,Fennell:2006lq} The GSF and TSF
approaches provide smooth truncation of energies, forces, and torques
at the real-space cutoff, and both converge to DSF electrostatics for
point-charge interactions.  The TSF model is based on a high-order
truncated Taylor expansion which can be relatively perturbative inside
the cutoff sphere.  The GSF model takes the gradient from an images of
the interacting multipole that has been projected onto the cutoff
sphere to derive shifted force and torque expressions, and is a
significantly more gentle approach.

The GSF method produced quantitative agreement with Ewald energy,
force, and torques.  It also performs well in conserving energy in MD
simulations.  The Taylor-shifted (TSF) model provides smooth dynamics,
but these take place on a potential energy surface that is
significantly perturbed from Ewald-based electrostatics.  Because it
performs relatively poorly compared with GSF, it may seem odd that
that the TSF model was included in this work.  However, the functional
forms derived for the SP and GSF methods depend on the separation of
orientational contributions that were made visible by the Taylor
series of the electrostatic kernel at the cutoff radius. The TSF
method also has the unique property that a large number of derivatives
can be made to vanish at the cutoff radius.  This property has proven
useful in past treatments of the corrections to the fluctuation
formula for dielectric constants.\cite{Izvekov:2008wo} 

Reproduction of both structural and dynamical features in the liquid
systems is remarkably good for both the SP and GSF models.  Pair
distribution functions are essentially equivalent to the same
functions produced using Ewald-based electrostatics, and with moderate
damping, a structural feature that directly probes the electrostatic
interaction (e.g. the mean electrostatic potential energy) can also be
made quantitative.  Dynamical features are sensitive probes of the
forces and torques produced by these methods, and even though the
smooth behavior of forces is produced by perturbing the overall
potential, the diffusion constants and orientational correlation times
are quite close to the Ewald-based results.

The only cases we have found where the new GSF and SP real-space
methods can be problematic are those which retain a bulk dipole moment
at large distances (e.g. the $Z_1$ dipolar lattice).  In ferroelectric
materials, uniform weighting of the orientational contributions can be
important for converging the total energy.  In these cases, the
damping function which causes the non-uniform weighting can be
replaced by the bare electrostatic kernel, and the energies return to
the expected converged values.

Based on the results of this work, we can conclude that the GSF method
is a suitable and efficient replacement for the Ewald sum for
evaluating electrostatic interactions in modern MD simulations, and
the SP meethod would be an excellent choice for Monte Carlo
simulations where smooth forces and energy conservation are not
important.  Both the SP and GSF methods retain excellent fidelity to
the Ewald energies, forces and torques.  Additionally, the energy
drift and fluctuations from the GSF electrostatics are significantly
better than a multipolar Ewald sum for finite-sized reciprocal spaces.

As in all purely pairwise cutoff methods, the SP, GSF and TSF methods
are expected to scale approximately {\it linearly} with system size,
and are easily parallelizable.  This should result in substantial
reductions in the computational cost of performing large simulations.
With the proper use of pre-computation and spline interpolation of the
radial functions, the real-space methods are essentially the same cost
as a simple real-space cutoff.  They require no Fourier transforms or
$k$-space sums, and guarantee the smooth handling of energies, forces,
and torques as multipoles cross the real-space cutoff boundary.

We are not suggesting that there is any flaw with the Ewald sum; in
fact, it is the standard by which the SP, GSF, and TSF methods have
been judged in this work.  However, these results provide evidence
that in the typical simulations performed today, the Ewald summation
may no longer be required to obtain the level of accuracy most
researchers have come to expect.

\begin{acknowledgments}
  JDG acknowledges helpful discussions with Christopher
  Fennell. Support for this project was provided by the National
  Science Foundation under grant CHE-1362211. Computational time was
  provided by the Center for Research Computing (CRC) at the
  University of Notre Dame.
\end{acknowledgments}

%\bibliographystyle{aip}
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\bibliography{references}
\end{document}

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