150 |
|
increasing time step. For each time step, the dotted line is total |
151 |
|
energy using the DLM integrator, and the solid line comes from the |
152 |
|
quaternion integrator. The larger time step plots are shifted up |
153 |
< |
from the true energy baseline for clarity.} \label{timestep} |
153 |
> |
from the true energy baseline for clarity.} |
154 |
> |
\label{methodFig:timestep} |
155 |
|
\end{figure} |
156 |
|
|
157 |
< |
In Fig.~\ref{timestep}, the resulting energy drift at various time |
158 |
< |
steps for both the DLM and quaternion integration schemes is |
159 |
< |
compared. All of the 1000 molecule water simulations started with |
160 |
< |
the same configuration, and the only difference was the method for |
161 |
< |
handling rotational motion. At time steps of 0.1 and 0.5 fs, both |
162 |
< |
methods for propagating molecule rotation conserve energy fairly |
163 |
< |
well, with the quaternion method showing a slight energy drift over |
164 |
< |
time in the 0.5 fs time step simulation. At time steps of 1 and 2 |
165 |
< |
fs, the energy conservation benefits of the DLM method are clearly |
166 |
< |
demonstrated. Thus, while maintaining the same degree of energy |
167 |
< |
conservation, one can take considerably longer time steps, leading |
168 |
< |
to an overall reduction in computation time. |
157 |
> |
In Fig.~\ref{methodFig:timestep}, the resulting energy drift at |
158 |
> |
various time steps for both the DLM and quaternion integration |
159 |
> |
schemes is compared. All of the 1000 molecule water simulations |
160 |
> |
started with the same configuration, and the only difference was the |
161 |
> |
method for handling rotational motion. At time steps of 0.1 and 0.5 |
162 |
> |
fs, both methods for propagating molecule rotation conserve energy |
163 |
> |
fairly well, with the quaternion method showing a slight energy |
164 |
> |
drift over time in the 0.5 fs time step simulation. At time steps of |
165 |
> |
1 and 2 fs, the energy conservation benefits of the DLM method are |
166 |
> |
clearly demonstrated. Thus, while maintaining the same degree of |
167 |
> |
energy conservation, one can take considerably longer time steps, |
168 |
> |
leading to an overall reduction in computation time. |
169 |
|
|
170 |
|
\subsection{\label{methodSection:NVT}Nos\'{e}-Hoover Thermostatting} |
171 |
|
|
172 |
< |
The Nos\'e-Hoover equations of motion are given by\cite{Hoover85} |
172 |
> |
The Nos\'e-Hoover equations of motion are given by\cite{Hoover1985} |
173 |
|
\begin{eqnarray} |
174 |
|
\dot{{\bf r}} & = & {\bf v}, \\ |
175 |
|
\dot{{\bf v}} & = & \frac{{\bf f}}{m} - \chi {\bf v} ,\\ |
285 |
|
|
286 |
|
The Nos\'e-Hoover algorithm is known to conserve a Hamiltonian for |
287 |
|
the extended system that is, to within a constant, identical to the |
288 |
< |
Helmholtz free energy,\cite{melchionna93} |
288 |
> |
Helmholtz free energy,\cite{Melchionna1993} |
289 |
|
\begin{equation} |
290 |
|
H_{\mathrm{NVT}} = V + K + f k_B T_{\mathrm{target}} \left( |
291 |
|
\frac{\tau_{T}^2 \chi^2(t)}{2} + \int_{0}^{t} \chi(t^\prime) |
296 |
|
last column of the {\tt .stat} file to allow checks on the quality |
297 |
|
of the integration. |
298 |
|
|
298 |
– |
Bond constraints are applied at the end of both the {\tt moveA} and |
299 |
– |
{\tt moveB} portions of the algorithm. Details on the constraint |
300 |
– |
algorithms are given in section \ref{oopseSec:rattle}. |
301 |
– |
|
299 |
|
\subsection{\label{methodSection:NPTi}Constant-pressure integration with |
300 |
|
isotropic box deformations (NPTi)} |
301 |
|
|
302 |
|
To carry out isobaric-isothermal ensemble calculations {\sc oopse} |
303 |
|
implements the Melchionna modifications to the |
304 |
< |
Nos\'e-Hoover-Andersen equations of motion,\cite{melchionna93} |
304 |
> |
Nos\'e-Hoover-Andersen equations of motion,\cite{Melchionna1993} |
305 |
|
|
306 |
|
\begin{eqnarray} |
307 |
|
\dot{{\bf r}} & = & {\bf v} + \eta \left( {\bf r} - {\bf R}_0 \right), \\ |
618 |
|
called the average surface area per lipid. Constat area and constant |
619 |
|
lateral pressure simulation can be achieved by extending the |
620 |
|
standard NPT ensemble with a different pressure control strategy |
621 |
+ |
|
622 |
|
\begin{equation} |
623 |
< |
\dot |
624 |
< |
\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\leftrightarrow$}} |
625 |
< |
\over \eta } _{\alpha \beta } = \left\{ \begin{array}{l} |
626 |
< |
\frac{{V(P_{\alpha \beta } - P_{{\rm{target}}} )}}{{\tau _{\rm{B}}^{\rm{2}} fk_B T_{{\rm{target}}} }}{\rm{ }}(\alpha = \beta = z), \\ |
627 |
< |
0{\rm{ }}(\alpha \ne z{\rm{ }}or{\rm{ }}\beta \ne z) \\ |
628 |
< |
\end{array} \right. |
631 |
< |
\label{methodEquation:NPATeta} |
623 |
> |
\dot {\overleftrightarrow{\eta}} _{\alpha \beta}=\left\{\begin{array}{ll} |
624 |
> |
\frac{{V(P_{\alpha \beta } - P_{{\rm{target}}} )}}{{\tau_{\rm{B}}^{\rm{2}} fk_B T_{{\rm{target}}} }} |
625 |
> |
& \mbox{if $ \alpha = \beta = z$}\\ |
626 |
> |
0 & \mbox{otherwise}\\ |
627 |
> |
\end{array} |
628 |
> |
\right. |
629 |
|
\end{equation} |
630 |
+ |
|
631 |
|
Note that the iterative schemes for NPAT are identical to those |
632 |
|
described for the NPTi integrator. |
633 |
|
|
646 |
|
pressure. The equation of motion for cell size control tensor, |
647 |
|
$\eta$, in $NP\gamma T$ is |
648 |
|
\begin{equation} |
649 |
< |
\dot |
650 |
< |
\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\leftrightarrow$}} |
651 |
< |
\over \eta } _{\alpha \beta } = \left\{ \begin{array}{l} |
652 |
< |
- A_{xy} (\gamma _\alpha - \gamma _{{\rm{target}}} ){\rm{ (}}\alpha = \beta = x{\rm{ or }} = y{\rm{)}} \\ |
653 |
< |
\frac{{V(P_{\alpha \beta } - P_{{\rm{target}}} )}}{{\tau _{\rm{B}}^{\rm{2}} fk_B T_{{\rm{target}}} }}{\rm{ }}(\alpha = \beta = z) \\ |
654 |
< |
0{\rm{ }}(\alpha \ne \beta ) \\ |
657 |
< |
\end{array} \right. |
658 |
< |
\label{methodEquation:NPrTeta} |
649 |
> |
\dot {\overleftrightarrow{\eta}} _{\alpha \beta}=\left\{\begin{array}{ll} |
650 |
> |
- A_{xy} (\gamma _\alpha - \gamma _{{\rm{target}}} ) & \mbox{$\alpha = \beta = x$ or $\alpha = \beta = y$}\\ |
651 |
> |
\frac{{V(P_{\alpha \beta } - P_{{\rm{target}}} )}}{{\tau _{\rm{B}}^{\rm{2}} fk_B T_{{\rm{target}}}}} & \mbox{$\alpha = \beta = z$} \\ |
652 |
> |
0 & \mbox{$\alpha \ne \beta$} \\ |
653 |
> |
\end{array} |
654 |
> |
\right. |
655 |
|
\end{equation} |
656 |
|
where $ \gamma _{{\rm{target}}}$ is the external surface tension and |
657 |
|
the instantaneous surface tensor $\gamma _\alpha$ is given by |
658 |
|
\begin{equation} |
659 |
< |
\gamma _\alpha = - h_z |
660 |
< |
(\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\leftrightarrow$}} |
665 |
< |
\over P} _{\alpha \alpha } - P_{{\rm{target}}} ) |
659 |
> |
\gamma _\alpha = - h_z( \overleftrightarrow{P} _{\alpha \alpha } |
660 |
> |
- P_{{\rm{target}}} ) |
661 |
|
\label{methodEquation:instantaneousSurfaceTensor} |
662 |
|
\end{equation} |
663 |
|
|