| 140 |
|
\end{split} |
| 141 |
|
\label{eq:EwaldSum} |
| 142 |
|
\end{equation} |
| 143 |
< |
where $\alpha$ is a damping parameter, or separation constant, with |
| 144 |
< |
units of \AA$^{-1}$, $\mathbf{k}$ are the reciprocal vectors and are |
| 145 |
< |
equal to $2\pi\mathbf{n}/L^2$, and $\epsilon_\textrm{S}$ is the |
| 146 |
< |
dielectric constant of the surrounding medium. The final two terms of |
| 143 |
> |
where $\alpha$ is the damping or convergence parameter with units of |
| 144 |
> |
\AA$^{-1}$, $\mathbf{k}$ are the reciprocal vectors and are equal to |
| 145 |
> |
$2\pi\mathbf{n}/L^2$, and $\epsilon_\textrm{S}$ is the dielectric |
| 146 |
> |
constant of the surrounding medium. The final two terms of |
| 147 |
|
eq. (\ref{eq:EwaldSum}) are a particle-self term and a dipolar term |
| 148 |
|
for interacting with a surrounding dielectric.\cite{Allen87} This |
| 149 |
|
dipolar term was neglected in early applications in molecular |
| 154 |
|
system is said to be using conducting (or ``tin-foil'') boundary |
| 155 |
|
conditions, $\epsilon_{\rm S} = \infty$. Figure |
| 156 |
|
\ref{fig:ewaldTime} shows how the Ewald sum has been applied over |
| 157 |
< |
time. Initially, due to the small sizes of the systems that could be |
| 158 |
< |
feasibly simulated, the entire simulation box was replicated to |
| 159 |
< |
convergence. In more modern simulations, the simulation boxes have |
| 160 |
< |
grown large enough that a real-space cutoff could potentially give |
| 161 |
< |
convergent behavior. Indeed, it has often been observed that the |
| 162 |
< |
reciprocal-space portion of the Ewald sum can be vanishingly |
| 163 |
< |
small compared to the real-space portion.\cite{XXX} |
| 157 |
> |
time. Initially, due to the small system sizes that could be |
| 158 |
> |
simulated feasibly, the entire simulation box was replicated to |
| 159 |
> |
convergence. In more modern simulations, the systems have grown large |
| 160 |
> |
enough that a real-space cutoff could potentially give convergent |
| 161 |
> |
behavior. Indeed, it has been observed that with the choice of a |
| 162 |
> |
small $\alpha$, the reciprocal-space portion of the Ewald sum can be |
| 163 |
> |
rapidly convergent and small relative to the real-space |
| 164 |
> |
portion.\cite{Karasawa89,Kolafa92} |
| 165 |
|
|
| 166 |
|
\begin{figure} |
| 167 |
|
\centering |
| 168 |
< |
\includegraphics[width = \linewidth]{./ewaldProgression.pdf} |
| 169 |
< |
\caption{How the application of the Ewald summation has changed with |
| 170 |
< |
the increase in computer power. Initially, only small numbers of |
| 171 |
< |
particles could be studied, and the Ewald sum acted to replicate the |
| 172 |
< |
unit cell charge distribution out to convergence. Now, much larger |
| 173 |
< |
systems of charges are investigated with fixed distance cutoffs. The |
| 173 |
< |
calculated structure factor is used to sum out to great distance, and |
| 174 |
< |
a surrounding dielectric term is included.} |
| 168 |
> |
\includegraphics[width = \linewidth]{./ewaldProgression2.pdf} |
| 169 |
> |
\caption{The change in the application of the Ewald sum with |
| 170 |
> |
increasing computational power. Initially, only small systems could |
| 171 |
> |
be studied, and the Ewald sum replicated the simulation box to |
| 172 |
> |
convergence. Now, much larger systems of charges are investigated |
| 173 |
> |
with fixed-distance cutoffs.} |
| 174 |
|
\label{fig:ewaldTime} |
| 175 |
|
\end{figure} |
| 176 |
|
|
| 177 |
|
The original Ewald summation is an $\mathscr{O}(N^2)$ algorithm. The |
| 178 |
< |
separation constant $(\alpha)$ plays an important role in balancing |
| 178 |
> |
convergence parameter $(\alpha)$ plays an important role in balancing |
| 179 |
|
the computational cost between the direct and reciprocal-space |
| 180 |
|
portions of the summation. The choice of this value allows one to |
| 181 |
|
select whether the real-space or reciprocal space portion of the |
| 491 |
|
techniques utilize pairwise summations of interactions between |
| 492 |
|
particle sites, but they use these summations in different ways. |
| 493 |
|
|
| 494 |
< |
In MC, the potential energy difference between two subsequent |
| 495 |
< |
configurations dictates the progression of MC sampling. Going back to |
| 496 |
< |
the origins of this method, the acceptance criterion for the canonical |
| 497 |
< |
ensemble laid out by Metropolis \textit{et al.} states that a |
| 498 |
< |
subsequent configuration is accepted if $\Delta E < 0$ or if $\xi < |
| 499 |
< |
\exp(-\Delta E/kT)$, where $\xi$ is a random number between 0 and |
| 500 |
< |
1.\cite{Metropolis53} Maintaining the correct $\Delta E$ when using an |
| 501 |
< |
alternate method for handling the long-range electrostatics will |
| 502 |
< |
ensure proper sampling from the ensemble. |
| 494 |
> |
In MC, the potential energy difference between configurations dictates |
| 495 |
> |
the progression of MC sampling. Going back to the origins of this |
| 496 |
> |
method, the acceptance criterion for the canonical ensemble laid out |
| 497 |
> |
by Metropolis \textit{et al.} states that a subsequent configuration |
| 498 |
> |
is accepted if $\Delta E < 0$ or if $\xi < \exp(-\Delta E/kT)$, where |
| 499 |
> |
$\xi$ is a random number between 0 and 1.\cite{Metropolis53} |
| 500 |
> |
Maintaining the correct $\Delta E$ when using an alternate method for |
| 501 |
> |
handling the long-range electrostatics will ensure proper sampling |
| 502 |
> |
from the ensemble. |
| 503 |
|
|
| 504 |
|
In MD, the derivative of the potential governs how the system will |
| 505 |
|
progress in time. Consequently, the force and torque vectors on each |
| 512 |
|
vectors will diverge from each other more rapidly. |
| 513 |
|
|
| 514 |
|
\subsection{Monte Carlo and the Energy Gap}\label{sec:MCMethods} |
| 515 |
+ |
|
| 516 |
|
The pairwise summation techniques (outlined in section |
| 517 |
|
\ref{sec:ESMethods}) were evaluated for use in MC simulations by |
| 518 |
|
studying the energy differences between conformations. We took the |
| 519 |
|
SPME-computed energy difference between two conformations to be the |
| 520 |
|
correct behavior. An ideal performance by an alternative method would |
| 521 |
< |
reproduce these energy differences exactly. Since none of the methods |
| 522 |
< |
provide exact energy differences, we used linear least squares |
| 523 |
< |
regressions of the $\Delta E$ values between configurations using SPME |
| 524 |
< |
against $\Delta E$ values using tested methods provides a quantitative |
| 525 |
< |
comparison of this agreement. Unitary results for both the |
| 526 |
< |
correlation and correlation coefficient for these regressions indicate |
| 527 |
< |
equivalent energetic results between the method under consideration |
| 528 |
< |
and electrostatics handled using SPME. Sample correlation plots for |
| 529 |
< |
two alternate methods are shown in Fig. \ref{fig:linearFit}. |
| 521 |
> |
reproduce these energy differences exactly (even if the absolute |
| 522 |
> |
energies calculated by the methods are different). Since none of the |
| 523 |
> |
methods provide exact energy differences, we used linear least squares |
| 524 |
> |
regressions of energy gap data to evaluate how closely the methods |
| 525 |
> |
mimicked the Ewald energy gaps. Unitary results for both the |
| 526 |
> |
correlation (slope) and correlation coefficient for these regressions |
| 527 |
> |
indicate perfect agreement between the alternative method and SPME. |
| 528 |
> |
Sample correlation plots for two alternate methods are shown in |
| 529 |
> |
Fig. \ref{fig:linearFit}. |
| 530 |
|
|
| 531 |
|
\begin{figure} |
| 532 |
|
\centering |
| 533 |
|
\includegraphics[width = \linewidth]{./dualLinear.pdf} |
| 534 |
< |
\caption{Example least squares regressions of the configuration energy differences for SPC/E water systems. The upper plot shows a data set with a poor correlation coefficient ($R^2$), while the lower plot shows a data set with a good correlation coefficient.} |
| 535 |
< |
\label{fig:linearFit} |
| 534 |
> |
\caption{Example least squares regressions of the configuration energy |
| 535 |
> |
differences for SPC/E water systems. The upper plot shows a data set |
| 536 |
> |
with a poor correlation coefficient ($R^2$), while the lower plot |
| 537 |
> |
shows a data set with a good correlation coefficient.} |
| 538 |
> |
\label{fig:linearFit} |
| 539 |
|
\end{figure} |
| 540 |
|
|
| 541 |
|
Each system type (detailed in section \ref{sec:RepSims}) was |
| 542 |
|
represented using 500 independent configurations. Additionally, we |
| 543 |
< |
used seven different system types, so each of the alternate |
| 543 |
> |
used seven different system types, so each of the alternative |
| 544 |
|
(non-Ewald) electrostatic summation methods was evaluated using |
| 545 |
|
873,250 configurational energy differences. |
| 546 |
|
|
| 570 |
|
investigated through measurement of the angle ($\theta$) formed |
| 571 |
|
between those computed from the particular method and those from SPME, |
| 572 |
|
\begin{equation} |
| 573 |
< |
\theta_f = \cos^{-1} \left(\hat{f}_\textrm{SPME} \cdot \hat{f}_\textrm{Method}\right), |
| 574 |
< |
\end{equation} |
| 575 |
< |
where $\hat{f}_\textrm{M}$ is the unit vector pointing along the |
| 576 |
< |
force vector computed using method $M$. |
| 577 |
< |
|
| 578 |
< |
Each of these $\theta$ values was accumulated in a distribution |
| 576 |
< |
function, weighted by the area on the unit sphere. Non-linear |
| 577 |
< |
Gaussian fits were used to measure the width of the resulting |
| 578 |
< |
distributions. |
| 579 |
< |
|
| 580 |
< |
\begin{figure} |
| 581 |
< |
\centering |
| 582 |
< |
\includegraphics[width = \linewidth]{./gaussFit.pdf} |
| 583 |
< |
\caption{Sample fit of the angular distribution of the force vectors over all of the studied systems. Gaussian fits were used to obtain values for the variance in force and torque vectors used in the following figure.} |
| 584 |
< |
\label{fig:gaussian} |
| 585 |
< |
\end{figure} |
| 586 |
< |
|
| 587 |
< |
Figure \ref{fig:gaussian} shows an example distribution with applied |
| 588 |
< |
non-linear fits. The solid line is a Gaussian profile, while the |
| 589 |
< |
dotted line is a Voigt profile, a convolution of a Gaussian and a |
| 590 |
< |
Lorentzian. Since this distribution is a measure of angular error |
| 573 |
> |
\theta_f = \cos^{-1} \left(\hat{F}_\textrm{SPME} \cdot \hat{F}_\textrm{M}\right), |
| 574 |
> |
\end{equation} |
| 575 |
> |
where $\hat{f}_\textrm{M}$ is the unit vector pointing along the force |
| 576 |
> |
vector computed using method M. Each of these $\theta$ values was |
| 577 |
> |
accumulated in a distribution function and weighted by the area on the |
| 578 |
> |
unit sphere. Since this distribution is a measure of angular error |
| 579 |
|
between two different electrostatic summation methods, there is no |
| 580 |
< |
{\it a priori} reason for the profile to adhere to any specific shape. |
| 581 |
< |
Gaussian fits was used to compare all the tested methods. The |
| 582 |
< |
variance ($\sigma^2$) was extracted from each of these fits and was |
| 583 |
< |
used to compare distribution widths. Values of $\sigma^2$ near zero |
| 584 |
< |
indicate vector directions indistinguishable from those calculated |
| 585 |
< |
when using the reference method (SPME). |
| 580 |
> |
{\it a priori} reason for the profile to adhere to any specific |
| 581 |
> |
shape. Thus, gaussian fits were used to measure the width of the |
| 582 |
> |
resulting distributions. |
| 583 |
> |
% |
| 584 |
> |
%\begin{figure} |
| 585 |
> |
%\centering |
| 586 |
> |
%\includegraphics[width = \linewidth]{./gaussFit.pdf} |
| 587 |
> |
%\caption{Sample fit of the angular distribution of the force vectors |
| 588 |
> |
%accumulated using all of the studied systems. Gaussian fits were used |
| 589 |
> |
%to obtain values for the variance in force and torque vectors.} |
| 590 |
> |
%\label{fig:gaussian} |
| 591 |
> |
%\end{figure} |
| 592 |
> |
% |
| 593 |
> |
%Figure \ref{fig:gaussian} shows an example distribution with applied |
| 594 |
> |
%non-linear fits. The solid line is a Gaussian profile, while the |
| 595 |
> |
%dotted line is a Voigt profile, a convolution of a Gaussian and a |
| 596 |
> |
%Lorentzian. |
| 597 |
> |
%Since this distribution is a measure of angular error between two |
| 598 |
> |
%different electrostatic summation methods, there is no {\it a priori} |
| 599 |
> |
%reason for the profile to adhere to any specific shape. |
| 600 |
> |
%Gaussian fits was used to compare all the tested methods. |
| 601 |
> |
The variance ($\sigma^2$) was extracted from each of these fits and |
| 602 |
> |
was used to compare distribution widths. Values of $\sigma^2$ near |
| 603 |
> |
zero indicate vector directions indistinguishable from those |
| 604 |
> |
calculated when using the reference method (SPME). |
| 605 |
|
|
| 606 |
|
\subsection{Short-time Dynamics} |
| 607 |
< |
Evaluation of the short-time dynamics of charged systems was performed |
| 608 |
< |
by considering the 1000 K NaCl crystal system while using a subset of the |
| 609 |
< |
best performing pairwise methods. The NaCl crystal was chosen to |
| 610 |
< |
avoid possible complications involving the propagation techniques of |
| 611 |
< |
orientational motion in molecular systems. All systems were started |
| 612 |
< |
with the same initial positions and velocities. Simulations were |
| 613 |
< |
performed under the microcanonical ensemble, and velocity |
| 607 |
> |
|
| 608 |
> |
The effects of the alternative electrostatic summation methods on the |
| 609 |
> |
short-time dynamics of charged systems were evaluated by considering a |
| 610 |
> |
NaCl crystal at a temperature of 1000 K. A subset of the best |
| 611 |
> |
performing pairwise methods was used in this comparison. The NaCl |
| 612 |
> |
crystal was chosen to avoid possible complications from the treatment |
| 613 |
> |
of orientational motion in molecular systems. All systems were |
| 614 |
> |
started with the same initial positions and velocities. Simulations |
| 615 |
> |
were performed under the microcanonical ensemble, and velocity |
| 616 |
|
autocorrelation functions (Eq. \ref{eq:vCorr}) were computed for each |
| 617 |
|
of the trajectories, |
| 618 |
|
\begin{equation} |
| 626 |
|
functions was used for comparisons. |
| 627 |
|
|
| 628 |
|
\subsection{Long-Time and Collective Motion}\label{sec:LongTimeMethods} |
| 629 |
< |
Evaluation of the long-time dynamics of charged systems was performed |
| 630 |
< |
by considering the NaCl crystal system, again while using a subset of |
| 631 |
< |
the best performing pairwise methods. To enhance the atomic motion, |
| 632 |
< |
these crystals were equilibrated at 1000 K, near the experimental |
| 633 |
< |
$T_m$ for NaCl. Simulations were performed under the microcanonical |
| 634 |
< |
ensemble, and velocity information was saved every 5 fs over 100 ps |
| 635 |
< |
trajectories. The power spectrum ($I(\omega)$) was obtained via |
| 627 |
< |
Fourier transform of the velocity autocorrelation function |
| 628 |
< |
\begin{equation} |
| 629 |
< |
I(\omega) = \frac{1}{2\pi}\int^{\infty}_{-\infty}C_v(t)e^{-i\omega t}dt, |
| 629 |
> |
|
| 630 |
> |
The effects of the same subset of alternative electrostatic methods on |
| 631 |
> |
the {\it long-time} dynamics of charged systems were evaluated using |
| 632 |
> |
the same model system (NaCl crystals at 1000K). The power spectrum |
| 633 |
> |
($I(\omega)$) was obtained via Fourier transform of the velocity |
| 634 |
> |
autocorrelation function, \begin{equation} I(\omega) = |
| 635 |
> |
\frac{1}{2\pi}\int^{\infty}_{-\infty}C_v(t)e^{-i\omega t}dt, |
| 636 |
|
\label{eq:powerSpec} |
| 637 |
|
\end{equation} |
| 638 |
|
where the frequency, $\omega=0,\ 1,\ ...,\ N-1$. Again, because the |
| 639 |
|
NaCl crystal is composed of two different atom types, the average of |
| 640 |
< |
the two resulting power spectra was used for comparisons. |
| 640 |
> |
the two resulting power spectra was used for comparisons. Simulations |
| 641 |
> |
were performed under the microcanonical ensemble, and velocity |
| 642 |
> |
information was saved every 5 fs over 100 ps trajectories. |
| 643 |
|
|
| 644 |
|
\subsection{Representative Simulations}\label{sec:RepSims} |
| 645 |
< |
A variety of common and representative simulations were analyzed to |
| 646 |
< |
determine the relative effectiveness of the pairwise summation |
| 647 |
< |
techniques in reproducing the energetics and dynamics exhibited by |
| 648 |
< |
SPME. The studied systems were as follows: |
| 645 |
> |
A variety of representative simulations were analyzed to determine the |
| 646 |
> |
relative effectiveness of the pairwise summation techniques in |
| 647 |
> |
reproducing the energetics and dynamics exhibited by SPME. We wanted |
| 648 |
> |
to span the space of modern simulations (i.e. from liquids of neutral |
| 649 |
> |
molecules to ionic crystals), so the systems studied were: |
| 650 |
|
\begin{enumerate} |
| 651 |
< |
\item Liquid Water |
| 652 |
< |
\item Crystalline Water (Ice I$_\textrm{c}$) |
| 653 |
< |
\item NaCl Crystal |
| 654 |
< |
\item NaCl Melt |
| 655 |
< |
\item Low Ionic Strength Solution of NaCl in Water |
| 656 |
< |
\item High Ionic Strength Solution of NaCl in Water |
| 657 |
< |
\item 6 \AA\ Radius Sphere of Argon in Water |
| 651 |
> |
\item liquid water (SPC/E),\cite{Berendsen87} |
| 652 |
> |
\item crystalline water (Ice I$_\textrm{c}$ crystals of SPC/E), |
| 653 |
> |
\item NaCl crystals, |
| 654 |
> |
\item NaCl melts, |
| 655 |
> |
\item a low ionic strength solution of NaCl in water (0.11 M), |
| 656 |
> |
\item a high ionic strength solution of NaCl in water (1.1 M), and |
| 657 |
> |
\item a 6 \AA\ radius sphere of Argon in water. |
| 658 |
|
\end{enumerate} |
| 659 |
|
By utilizing the pairwise techniques (outlined in section |
| 660 |
|
\ref{sec:ESMethods}) in systems composed entirely of neutral groups, |
| 661 |
< |
charged particles, and mixtures of the two, we can comment on possible |
| 662 |
< |
system dependence and/or universal applicability of the techniques. |
| 661 |
> |
charged particles, and mixtures of the two, we hope to discern under |
| 662 |
> |
which conditions it will be possible to use one of the alternative |
| 663 |
> |
summation methodologies instead of the Ewald sum. |
| 664 |
|
|
| 665 |
< |
Generation of the system configurations was dependent on the system |
| 666 |
< |
type. For the solid and liquid water configurations, configuration |
| 667 |
< |
snapshots were taken at regular intervals from higher temperature 1000 |
| 668 |
< |
SPC/E water molecule trajectories and each equilibrated |
| 669 |
< |
individually.\cite{Berendsen87} The solid and liquid NaCl systems |
| 670 |
< |
consisted of 500 Na+ and 500 Cl- ions and were selected and |
| 671 |
< |
equilibrated in the same fashion as the water systems. For the low |
| 672 |
< |
and high ionic strength NaCl solutions, 4 and 40 ions were first |
| 673 |
< |
solvated in a 1000 water molecule boxes respectively. Ion and water |
| 674 |
< |
positions were then randomly swapped, and the resulting configurations |
| 675 |
< |
were again equilibrated individually. Finally, for the Argon/Water |
| 676 |
< |
"charge void" systems, the identities of all the SPC/E waters within 6 |
| 677 |
< |
\AA\ of the center of the equilibrated water configurations were |
| 678 |
< |
converted to argon (Fig. \ref{fig:argonSlice}). |
| 665 |
> |
For the solid and liquid water configurations, configurations were |
| 666 |
> |
taken at regular intervals from high temperature trajectories of 1000 |
| 667 |
> |
SPC/E water molecules. Each configuration was equilibrated |
| 668 |
> |
independently at a lower temperature (300~K for the liquid, 200~K for |
| 669 |
> |
the crystal). The solid and liquid NaCl systems consisted of 500 |
| 670 |
> |
$\textrm{Na}^{+}$ and 500 $\textrm{Cl}^{-}$ ions. Configurations for |
| 671 |
> |
these systems were selected and equilibrated in the same manner as the |
| 672 |
> |
water systems. The equilibrated temperatures were 1000~K for the NaCl |
| 673 |
> |
crystal and 7000~K for the liquid. The ionic solutions were made by |
| 674 |
> |
solvating 4 (or 40) ions in a periodic box containing 1000 SPC/E water |
| 675 |
> |
molecules. Ion and water positions were then randomly swapped, and |
| 676 |
> |
the resulting configurations were again equilibrated individually. |
| 677 |
> |
Finally, for the Argon / Water ``charge void'' systems, the identities |
| 678 |
> |
of all the SPC/E waters within 6 \AA\ of the center of the |
| 679 |
> |
equilibrated water configurations were converted to argon. |
| 680 |
> |
%(Fig. \ref{fig:argonSlice}). |
| 681 |
|
|
| 682 |
< |
\begin{figure} |
| 683 |
< |
\centering |
| 684 |
< |
\includegraphics[width = \linewidth]{./slice.pdf} |
| 685 |
< |
\caption{A slice from the center of a water box used in a charge void simulation. The darkened region represents the boundary sphere within which the water molecules were converted to argon atoms.} |
| 674 |
< |
\label{fig:argonSlice} |
| 675 |
< |
\end{figure} |
| 682 |
> |
These procedures guaranteed us a set of representative configurations |
| 683 |
> |
from chemically-relevant systems sampled from appropriate |
| 684 |
> |
ensembles. Force field parameters for the ions and Argon were taken |
| 685 |
> |
from the force field utilized by {\sc oopse}.\cite{Meineke05} |
| 686 |
|
|
| 687 |
< |
\subsection{Electrostatic Summation Methods}\label{sec:ESMethods} |
| 688 |
< |
Electrostatic summation method comparisons were performed using SPME, |
| 689 |
< |
the {\sc sp} and {\sc sf} methods - both with damping |
| 690 |
< |
parameters ($\alpha$) of 0.0, 0.1, 0.2, and 0.3 \AA$^{-1}$ (no, weak, |
| 691 |
< |
moderate, and strong damping respectively), reaction field with an |
| 692 |
< |
infinite dielectric constant, and an unmodified cutoff. Group-based |
| 693 |
< |
cutoffs with a fifth-order polynomial switching function were |
| 694 |
< |
necessary for the reaction field simulations and were utilized in the |
| 685 |
< |
SP, SF, and pure cutoff methods for comparison to the standard lack of |
| 686 |
< |
group-based cutoffs with a hard truncation. The SPME calculations |
| 687 |
< |
were performed using the TINKER implementation of SPME,\cite{Ponder87} |
| 688 |
< |
while all other method calculations were performed using the OOPSE |
| 689 |
< |
molecular mechanics package.\cite{Meineke05} |
| 687 |
> |
%\begin{figure} |
| 688 |
> |
%\centering |
| 689 |
> |
%\includegraphics[width = \linewidth]{./slice.pdf} |
| 690 |
> |
%\caption{A slice from the center of a water box used in a charge void |
| 691 |
> |
%simulation. The darkened region represents the boundary sphere within |
| 692 |
> |
%which the water molecules were converted to argon atoms.} |
| 693 |
> |
%\label{fig:argonSlice} |
| 694 |
> |
%\end{figure} |
| 695 |
|
|
| 696 |
< |
These methods were additionally evaluated with three different cutoff |
| 697 |
< |
radii (9, 12, and 15 \AA) to investigate possible cutoff radius |
| 698 |
< |
dependence. It should be noted that the damping parameter chosen in |
| 699 |
< |
SPME, or so called ``Ewald Coefficient", has a significant effect on |
| 700 |
< |
the energies and forces calculated. Typical molecular mechanics |
| 701 |
< |
packages default this to a value dependent on the cutoff radius and a |
| 702 |
< |
tolerance (typically less than $1 \times 10^{-4}$ kcal/mol). Smaller |
| 703 |
< |
tolerances are typically associated with increased accuracy, but this |
| 704 |
< |
usually means more time spent calculating the reciprocal-space portion |
| 705 |
< |
of the summation.\cite{Perram88,Essmann95} The default TINKER |
| 706 |
< |
tolerance of $1 \times 10^{-8}$ kcal/mol was used in all SPME |
| 707 |
< |
calculations, resulting in Ewald Coefficients of 0.4200, 0.3119, and |
| 708 |
< |
0.2476 \AA$^{-1}$ for cutoff radii of 9, 12, and 15 \AA\ respectively. |
| 696 |
> |
\subsection{Comparison of Summation Methods}\label{sec:ESMethods} |
| 697 |
> |
We compared the following alternative summation methods with results |
| 698 |
> |
from the reference method (SPME): |
| 699 |
> |
\begin{itemize} |
| 700 |
> |
\item {\sc sp} with damping parameters ($\alpha$) of 0.0, 0.1, 0.2, |
| 701 |
> |
and 0.3 \AA$^{-1}$, |
| 702 |
> |
\item {\sc sf} with damping parameters ($\alpha$) of 0.0, 0.1, 0.2, |
| 703 |
> |
and 0.3 \AA$^{-1}$, |
| 704 |
> |
\item reaction field with an infinite dielectric constant, and |
| 705 |
> |
\item an unmodified cutoff. |
| 706 |
> |
\end{itemize} |
| 707 |
> |
Group-based cutoffs with a fifth-order polynomial switching function |
| 708 |
> |
were utilized for the reaction field simulations. Additionally, we |
| 709 |
> |
investigated the use of these cutoffs with the SP, SF, and pure |
| 710 |
> |
cutoff. The SPME electrostatics were performed using the TINKER |
| 711 |
> |
implementation of SPME,\cite{Ponder87} while all other calculations |
| 712 |
> |
were performed using the {\sc oopse} molecular mechanics |
| 713 |
> |
package.\cite{Meineke05} All other portions of the energy calculation |
| 714 |
> |
(i.e. Lennard-Jones interactions) were handled in exactly the same |
| 715 |
> |
manner across all systems and configurations. |
| 716 |
|
|
| 717 |
+ |
The althernative methods were also evaluated with three different |
| 718 |
+ |
cutoff radii (9, 12, and 15 \AA). As noted previously, the |
| 719 |
+ |
convergence parameter ($\alpha$) plays a role in the balance of the |
| 720 |
+ |
real-space and reciprocal-space portions of the Ewald calculation. |
| 721 |
+ |
Typical molecular mechanics packages set this to a value dependent on |
| 722 |
+ |
the cutoff radius and a tolerance (typically less than $1 \times |
| 723 |
+ |
10^{-4}$ kcal/mol). Smaller tolerances are typically associated with |
| 724 |
+ |
increasing accuracy at the expense of computational time spent on the |
| 725 |
+ |
reciprocal-space portion of the summation.\cite{Perram88,Essmann95} |
| 726 |
+ |
The default TINKER tolerance of $1 \times 10^{-8}$ kcal/mol was used |
| 727 |
+ |
in all SPME calculations, resulting in Ewald coefficients of 0.4200, |
| 728 |
+ |
0.3119, and 0.2476 \AA$^{-1}$ for cutoff radii of 9, 12, and 15 \AA\ |
| 729 |
+ |
respectively. |
| 730 |
+ |
|
| 731 |
|
\section{Results and Discussion} |
| 732 |
|
|
| 733 |
|
\subsection{Configuration Energy Differences}\label{sec:EnergyResults} |
| 740 |
|
\begin{figure} |
| 741 |
|
\centering |
| 742 |
|
\includegraphics[width=5.5in]{./delEplot.pdf} |
| 743 |
< |
\caption{Statistical analysis of the quality of configurational energy differences for a given electrostatic method compared with the reference Ewald sum. Results with a value equal to 1 (dashed line) indicate $\Delta E$ values indistinguishable from those obtained using SPME. Different values of the cutoff radius are indicated with different symbols (9\AA\ = circles, 12\AA\ = squares, and 15\AA\ = inverted triangles).} |
| 743 |
> |
\caption{Statistical analysis of the quality of configurational energy |
| 744 |
> |
differences for a given electrostatic method compared with the |
| 745 |
> |
reference Ewald sum. Results with a value equal to 1 (dashed line) |
| 746 |
> |
indicate $\Delta E$ values indistinguishable from those obtained using |
| 747 |
> |
SPME. Different values of the cutoff radius are indicated with |
| 748 |
> |
different symbols (9\AA\ = circles, 12\AA\ = squares, and 15\AA\ = |
| 749 |
> |
inverted triangles).} |
| 750 |
|
\label{fig:delE} |
| 751 |
|
\end{figure} |
| 752 |
|
|
| 753 |
< |
In this figure, it is apparent that it is unreasonable to expect |
| 754 |
< |
realistic results using an unmodified cutoff. This is not all that |
| 755 |
< |
surprising since this results in large energy fluctuations as atoms or |
| 756 |
< |
molecules move in and out of the cutoff radius.\cite{Rahman71,Adams79} |
| 757 |
< |
These fluctuations can be alleviated to some degree by using group |
| 758 |
< |
based cutoffs with a switching |
| 727 |
< |
function.\cite{Adams79,Steinbach94,Leach01} The Group Switch Cutoff |
| 728 |
< |
row doesn't show a significant improvement in this plot because the |
| 729 |
< |
salt and salt solution systems contain non-neutral groups, see the |
| 730 |
< |
accompanying supporting information for a comparison where all groups |
| 731 |
< |
are neutral. |
| 753 |
> |
The most striking feature of this plot is how well the Shifted Force |
| 754 |
> |
({\sc sf}) and Shifted Potential ({\sc sp}) methods capture the energy |
| 755 |
> |
differences. For the undamped {\sc sf} method, and the |
| 756 |
> |
moderately-damped {\sc sp} methods, the results are nearly |
| 757 |
> |
indistinguishable from the Ewald results. The other common methods do |
| 758 |
> |
significantly less well. |
| 759 |
|
|
| 760 |
< |
Correcting the resulting charged cutoff sphere is one of the purposes |
| 761 |
< |
of the damped Coulomb summation proposed by Wolf \textit{et |
| 762 |
< |
al.},\cite{Wolf99} and this correction indeed improves the results as |
| 763 |
< |
seen in the {\sc sp} rows. While the undamped case of this |
| 764 |
< |
method is a significant improvement over the pure cutoff, it still |
| 765 |
< |
doesn't correlate that well with SPME. Inclusion of potential damping |
| 766 |
< |
improves the results, and using an $\alpha$ of 0.2 \AA $^{-1}$ shows |
| 767 |
< |
an excellent correlation and quality of fit with the SPME results, |
| 768 |
< |
particularly with a cutoff radius greater than 12 \AA . Use of a |
| 769 |
< |
larger damping parameter is more helpful for the shortest cutoff |
| 743 |
< |
shown, but it has a detrimental effect on simulations with larger |
| 744 |
< |
cutoffs. In the {\sc sf} sets, increasing damping results in |
| 745 |
< |
progressively poorer correlation. Overall, the undamped case is the |
| 746 |
< |
best performing set, as the correlation and quality of fits are |
| 747 |
< |
consistently superior regardless of the cutoff distance. This result |
| 748 |
< |
is beneficial in that the undamped case is less computationally |
| 749 |
< |
prohibitive do to the lack of complimentary error function calculation |
| 750 |
< |
when performing the electrostatic pair interaction. The reaction |
| 751 |
< |
field results illustrates some of that method's limitations, primarily |
| 752 |
< |
that it was developed for use in homogenous systems; although it does |
| 753 |
< |
provide results that are an improvement over those from an unmodified |
| 754 |
< |
cutoff. |
| 760 |
> |
The unmodified cutoff method is essentially unusable. This is not |
| 761 |
> |
surprising since hard cutoffs give large energy fluctuations as atoms |
| 762 |
> |
or molecules move in and out of the cutoff |
| 763 |
> |
radius.\cite{Rahman71,Adams79} These fluctuations can be alleviated to |
| 764 |
> |
some degree by using group based cutoffs with a switching |
| 765 |
> |
function.\cite{Adams79,Steinbach94,Leach01} However, we do not see |
| 766 |
> |
significant improvement using the group-switched cutoff because the |
| 767 |
> |
salt and salt solution systems contain non-neutral groups. Interested |
| 768 |
> |
readers can consult the accompanying supporting information for a |
| 769 |
> |
comparison where all groups are neutral. |
| 770 |
|
|
| 771 |
+ |
For the {\sc sp} method, inclusion of electrostatic damping improves |
| 772 |
+ |
the agreement with Ewald, and using an $\alpha$ of 0.2 \AA $^{-1}$ |
| 773 |
+ |
shows an excellent correlation and quality of fit with the SPME |
| 774 |
+ |
results, particularly with a cutoff radius greater than 12 |
| 775 |
+ |
\AA . Use of a larger damping parameter is more helpful for the |
| 776 |
+ |
shortest cutoff shown, but it has a detrimental effect on simulations |
| 777 |
+ |
with larger cutoffs. |
| 778 |
+ |
|
| 779 |
+ |
In the {\sc sf} sets, increasing damping results in progressively {\it |
| 780 |
+ |
worse} correlation with Ewald. Overall, the undamped case is the best |
| 781 |
+ |
performing set, as the correlation and quality of fits are |
| 782 |
+ |
consistently superior regardless of the cutoff distance. The undamped |
| 783 |
+ |
case is also less computationally demanding (because no evaluation of |
| 784 |
+ |
the complementary error function is required). |
| 785 |
+ |
|
| 786 |
+ |
The reaction field results illustrates some of that method's |
| 787 |
+ |
limitations, primarily that it was developed for use in homogenous |
| 788 |
+ |
systems; although it does provide results that are an improvement over |
| 789 |
+ |
those from an unmodified cutoff. |
| 790 |
+ |
|
| 791 |
|
\subsection{Magnitudes of the Force and Torque Vectors} |
| 792 |
|
|
| 793 |
|
Evaluation of pairwise methods for use in Molecular Dynamics |
| 794 |
|
simulations requires consideration of effects on the forces and |
| 795 |
< |
torques. Investigation of the force and torque vector magnitudes |
| 796 |
< |
provides a measure of the strength of these values relative to SPME. |
| 797 |
< |
Figures \ref{fig:frcMag} and \ref{fig:trqMag} respectively show the |
| 798 |
< |
force and torque vector magnitude regression results for the |
| 764 |
< |
accumulated analysis over all the system types. |
| 795 |
> |
torques. Figures \ref{fig:frcMag} and \ref{fig:trqMag} show the |
| 796 |
> |
regression results for the force and torque vector magnitudes, |
| 797 |
> |
respectively. The data in these figures was generated from an |
| 798 |
> |
accumulation of the statistics from all of the system types. |
| 799 |
|
|
| 800 |
|
\begin{figure} |
| 801 |
|
\centering |
| 802 |
|
\includegraphics[width=5.5in]{./frcMagplot.pdf} |
| 803 |
< |
\caption{Statistical analysis of the quality of the force vector magnitudes for a given electrostatic method compared with the reference Ewald sum. Results with a value equal to 1 (dashed line) indicate force magnitude values indistinguishable from those obtained using SPME. Different values of the cutoff radius are indicated with different symbols (9\AA\ = circles, 12\AA\ = squares, and 15\AA\ = inverted triangles).} |
| 803 |
> |
\caption{Statistical analysis of the quality of the force vector |
| 804 |
> |
magnitudes for a given electrostatic method compared with the |
| 805 |
> |
reference Ewald sum. Results with a value equal to 1 (dashed line) |
| 806 |
> |
indicate force magnitude values indistinguishable from those obtained |
| 807 |
> |
using SPME. Different values of the cutoff radius are indicated with |
| 808 |
> |
different symbols (9\AA\ = circles, 12\AA\ = squares, and 15\AA\ = |
| 809 |
> |
inverted triangles).} |
| 810 |
|
\label{fig:frcMag} |
| 811 |
|
\end{figure} |
| 812 |
|
|
| 813 |
+ |
Again, it is striking how well the Shifted Potential and Shifted Force |
| 814 |
+ |
methods are doing at reproducing the SPME forces. The undamped and |
| 815 |
+ |
weakly-damped {\sc sf} method gives the best agreement with Ewald. |
| 816 |
+ |
This is perhaps expected because this method explicitly incorporates a |
| 817 |
+ |
smooth transition in the forces at the cutoff radius as well as the |
| 818 |
+ |
neutralizing image charges. |
| 819 |
+ |
|
| 820 |
|
Figure \ref{fig:frcMag}, for the most part, parallels the results seen |
| 821 |
|
in the previous $\Delta E$ section. The unmodified cutoff results are |
| 822 |
|
poor, but using group based cutoffs and a switching function provides |
| 823 |
< |
a improvement much more significant than what was seen with $\Delta |
| 824 |
< |
E$. Looking at the {\sc sp} sets, the slope and $R^2$ |
| 825 |
< |
improve with the use of damping to an optimal result of 0.2 \AA |
| 826 |
< |
$^{-1}$ for the 12 and 15 \AA\ cutoffs. Further increases in damping, |
| 823 |
> |
an improvement much more significant than what was seen with $\Delta |
| 824 |
> |
E$. |
| 825 |
> |
|
| 826 |
> |
With moderate damping and a large enough cutoff radius, the {\sc sp} |
| 827 |
> |
method is generating usable forces. Further increases in damping, |
| 828 |
|
while beneficial for simulations with a cutoff radius of 9 \AA\ , is |
| 829 |
< |
detrimental to simulations with larger cutoff radii. The undamped |
| 830 |
< |
{\sc sf} method gives forces in line with those obtained using |
| 831 |
< |
SPME, and use of a damping function results in minor improvement. The |
| 784 |
< |
reaction field results are surprisingly good, considering the poor |
| 829 |
> |
detrimental to simulations with larger cutoff radii. |
| 830 |
> |
|
| 831 |
> |
The reaction field results are surprisingly good, considering the poor |
| 832 |
|
quality of the fits for the $\Delta E$ results. There is still a |
| 833 |
< |
considerable degree of scatter in the data, but it correlates well in |
| 834 |
< |
general. To be fair, we again note that the reaction field |
| 835 |
< |
calculations do not encompass NaCl crystal and melt systems, so these |
| 833 |
> |
considerable degree of scatter in the data, but the forces correlate |
| 834 |
> |
well with the Ewald forces in general. We note that the reaction |
| 835 |
> |
field calculations do not include the pure NaCl systems, so these |
| 836 |
|
results are partly biased towards conditions in which the method |
| 837 |
|
performs more favorably. |
| 838 |
|
|
| 839 |
|
\begin{figure} |
| 840 |
|
\centering |
| 841 |
|
\includegraphics[width=5.5in]{./trqMagplot.pdf} |
| 842 |
< |
\caption{Statistical analysis of the quality of the torque vector magnitudes for a given electrostatic method compared with the reference Ewald sum. Results with a value equal to 1 (dashed line) indicate torque magnitude values indistinguishable from those obtained using SPME. Different values of the cutoff radius are indicated with different symbols (9\AA\ = circles, 12\AA\ = squares, and 15\AA\ = inverted triangles).} |
| 842 |
> |
\caption{Statistical analysis of the quality of the torque vector |
| 843 |
> |
magnitudes for a given electrostatic method compared with the |
| 844 |
> |
reference Ewald sum. Results with a value equal to 1 (dashed line) |
| 845 |
> |
indicate torque magnitude values indistinguishable from those obtained |
| 846 |
> |
using SPME. Different values of the cutoff radius are indicated with |
| 847 |
> |
different symbols (9\AA\ = circles, 12\AA\ = squares, and 15\AA\ = |
| 848 |
> |
inverted triangles).} |
| 849 |
|
\label{fig:trqMag} |
| 850 |
|
\end{figure} |
| 851 |
|
|
| 852 |
< |
To evaluate the torque vector magnitudes, the data set from which |
| 853 |
< |
values are drawn is limited to rigid molecules in the systems |
| 854 |
< |
(i.e. water molecules). In spite of this smaller sampling pool, the |
| 802 |
< |
torque vector magnitude results in figure \ref{fig:trqMag} are still |
| 803 |
< |
similar to those seen for the forces; however, they more clearly show |
| 804 |
< |
the improved behavior that comes with increasing the cutoff radius. |
| 805 |
< |
Moderate damping is beneficial to the {\sc sp} and helpful |
| 806 |
< |
yet possibly unnecessary with the {\sc sf} method, and they also |
| 807 |
< |
show that over-damping adversely effects all cutoff radii rather than |
| 808 |
< |
showing an improvement for systems with short cutoffs. The reaction |
| 809 |
< |
field method performs well when calculating the torques, better than |
| 810 |
< |
the Shifted Force method over this limited data set. |
| 852 |
> |
Molecular torques were only available from the systems which contained |
| 853 |
> |
rigid molecules (i.e. the systems containing water). The data in |
| 854 |
> |
fig. \ref{fig:trqMag} is taken from this smaller sampling pool. |
| 855 |
|
|
| 856 |
+ |
Torques appear to be much more sensitive to charges at a longer |
| 857 |
+ |
distance. The striking feature in comparing the new electrostatic |
| 858 |
+ |
methods with SPME is how much the agreement improves with increasing |
| 859 |
+ |
cutoff radius. Again, the weakly damped and undamped {\sc sf} method |
| 860 |
+ |
appears to be reproducing the SPME torques most accurately. |
| 861 |
+ |
|
| 862 |
+ |
Water molecules are dipolar, and the reaction field method reproduces |
| 863 |
+ |
the effect of the surrounding polarized medium on each of the |
| 864 |
+ |
molecular bodies. Therefore it is not surprising that reaction field |
| 865 |
+ |
performs best of all of the methods on molecular torques. |
| 866 |
+ |
|
| 867 |
|
\subsection{Directionality of the Force and Torque Vectors} |
| 868 |
|
|
| 869 |
< |
Having force and torque vectors with magnitudes that are well |
| 870 |
< |
correlated to SPME is good, but if they are not pointing in the proper |
| 871 |
< |
direction the results will be incorrect. These vector directions were |
| 872 |
< |
investigated through measurement of the angle formed between them and |
| 873 |
< |
those from SPME. The results (Fig. \ref{fig:frcTrqAng}) are compared |
| 874 |
< |
through the variance ($\sigma^2$) of the Gaussian fits of the angle |
| 875 |
< |
error distributions of the combined set over all system types. |
| 869 |
> |
It is clearly important that a new electrostatic method can reproduce |
| 870 |
> |
the magnitudes of the force and torque vectors obtained via the Ewald |
| 871 |
> |
sum. However, the {\it directionality} of these vectors will also be |
| 872 |
> |
vital in calculating dynamical quantities accurately. Force and |
| 873 |
> |
torque directionalities were investigated by measuring the angles |
| 874 |
> |
formed between these vectors and the same vectors calculated using |
| 875 |
> |
SPME. The results (Fig. \ref{fig:frcTrqAng}) are compared through the |
| 876 |
> |
variance ($\sigma^2$) of the Gaussian fits of the angle error |
| 877 |
> |
distributions of the combined set over all system types. |
| 878 |
|
|
| 879 |
|
\begin{figure} |
| 880 |
|
\centering |
| 881 |
|
\includegraphics[width=5.5in]{./frcTrqAngplot.pdf} |
| 882 |
< |
\caption{Statistical analysis of the quality of the Gaussian fit of the force and torque vector angular distributions for a given electrostatic method compared with the reference Ewald sum. Results with a variance ($\sigma^2$) equal to zero (dashed line) indicate force and torque directions indistinguishable from those obtained using SPME. Different values of the cutoff radius are indicated with different symbols (9\AA\ = circles, 12\AA\ = squares, and 15\AA\ = inverted triangles).} |
| 882 |
> |
\caption{Statistical analysis of the width of the angular distribution |
| 883 |
> |
that the force and torque vectors from a given electrostatic method |
| 884 |
> |
make with their counterparts obtained using the reference Ewald sum. |
| 885 |
> |
Results with a variance ($\sigma^2$) equal to zero (dashed line) |
| 886 |
> |
indicate force and torque directions indistinguishable from those |
| 887 |
> |
obtained using SPME. Different values of the cutoff radius are |
| 888 |
> |
indicated with different symbols (9\AA\ = circles, 12\AA\ = squares, |
| 889 |
> |
and 15\AA\ = inverted triangles).} |
| 890 |
|
\label{fig:frcTrqAng} |
| 891 |
|
\end{figure} |
| 892 |
|
|
| 893 |
|
Both the force and torque $\sigma^2$ results from the analysis of the |
| 894 |
|
total accumulated system data are tabulated in figure |
| 895 |
< |
\ref{fig:frcTrqAng}. All of the sets, aside from the over-damped case |
| 896 |
< |
show the improvement afforded by choosing a longer simulation cutoff. |
| 897 |
< |
Increasing the cutoff from 9 to 12 \AA\ typically results in a halving |
| 898 |
< |
of the distribution widths, with a similar improvement going from 12 |
| 899 |
< |
to 15 \AA . The undamped {\sc sf}, Group Based Cutoff, and |
| 836 |
< |
Reaction Field methods all do equivalently well at capturing the |
| 837 |
< |
direction of both the force and torque vectors. Using damping |
| 838 |
< |
improves the angular behavior significantly for the {\sc sp} |
| 839 |
< |
and moderately for the {\sc sf} methods. Increasing the damping |
| 840 |
< |
too far is destructive for both methods, particularly to the torque |
| 841 |
< |
vectors. Again it is important to recognize that the force vectors |
| 842 |
< |
cover all particles in the systems, while torque vectors are only |
| 843 |
< |
available for neutral molecular groups. Damping appears to have a |
| 844 |
< |
more beneficial effect on non-neutral bodies, and this observation is |
| 845 |
< |
investigated further in the accompanying supporting information. |
| 895 |
> |
\ref{fig:frcTrqAng}. Here it is clear that the Shifted Potential ({\sc |
| 896 |
> |
sp}) method would be essentially unusable for molecular dynamics until |
| 897 |
> |
the damping function is added. The Shifted Force ({\sc sf}) method, |
| 898 |
> |
however, is generating force and torque vectors which are within a few |
| 899 |
> |
degrees of the Ewald results even with weak (or no) damping. |
| 900 |
|
|
| 901 |
+ |
All of the sets (aside from the over-damped case) show the improvement |
| 902 |
+ |
afforded by choosing a larger cutoff radius. Increasing the cutoff |
| 903 |
+ |
from 9 to 12 \AA\ typically results in a halving of the width of the |
| 904 |
+ |
distribution, with a similar improvement going from 12 to 15 |
| 905 |
+ |
\AA . |
| 906 |
+ |
|
| 907 |
+ |
The undamped {\sc sf}, group-based cutoff, and reaction field methods |
| 908 |
+ |
all do equivalently well at capturing the direction of both the force |
| 909 |
+ |
and torque vectors. Using damping improves the angular behavior |
| 910 |
+ |
significantly for the {\sc sp} and moderately for the {\sc sf} |
| 911 |
+ |
methods. Overdamping is detrimental to both methods. Again it is |
| 912 |
+ |
important to recognize that the force vectors cover all particles in |
| 913 |
+ |
the systems, while torque vectors are only available for neutral |
| 914 |
+ |
molecular groups. Damping appears to have a more beneficial effect on |
| 915 |
+ |
charged bodies, and this observation is investigated further in the |
| 916 |
+ |
accompanying supporting information. |
| 917 |
+ |
|
| 918 |
+ |
Although not discussed previously, group based cutoffs can be applied |
| 919 |
+ |
to both the {\sc sp} and {\sc sf} methods. Use of a switching |
| 920 |
+ |
function corrects for the discontinuities that arise when atoms of the |
| 921 |
+ |
two groups exit the cutoff radius before the group centers leave each |
| 922 |
+ |
other's cutoff. Though there are no significant benefits or drawbacks |
| 923 |
+ |
observed in $\Delta E$ and vector magnitude results when doing this, |
| 924 |
+ |
there is a measurable improvement in the vector angle results. Table |
| 925 |
+ |
\ref{tab:groupAngle} shows the angular variance values obtained using |
| 926 |
+ |
group based cutoffs and a switching function alongside the results |
| 927 |
+ |
seen in figure \ref{fig:frcTrqAng}. The {\sc sp} shows much narrower |
| 928 |
+ |
angular distributions for both the force and torque vectors when using |
| 929 |
+ |
an $\alpha$ of 0.2 \AA$^{-1}$ or less, while {\sc sf} shows |
| 930 |
+ |
improvements in the undamped and lightly damped cases. Thus, by |
| 931 |
+ |
calculating the electrostatic interactions in terms of molecular pairs |
| 932 |
+ |
rather than atomic pairs, the direction of the force and torque |
| 933 |
+ |
vectors can be determined more accurately. |
| 934 |
+ |
|
| 935 |
|
\begin{table}[htbp] |
| 936 |
|
\centering |
| 937 |
< |
\caption{Variance ($\sigma^2$) of the force (top set) and torque (bottom set) vector angle difference distributions for the Shifted Potential and Shifted Force methods. Calculations were performed both with (Y) and without (N) group based cutoffs and a switching function. The $\alpha$ values have units of \AA$^{-1}$ and the variance values have units of degrees$^2$.} |
| 937 |
> |
\caption{Variance ($\sigma^2$) of the force (top set) and torque |
| 938 |
> |
(bottom set) vector angle difference distributions for the Shifted Potential and Shifted Force methods. Calculations were performed both with (Y) and without (N) group based cutoffs and a switching function. The $\alpha$ values have units of \AA$^{-1}$ and the variance values have units of degrees$^2$.} |
| 939 |
|
\begin{tabular}{@{} ccrrrrrrrr @{}} |
| 940 |
|
\\ |
| 941 |
|
\toprule |
| 966 |
|
\label{tab:groupAngle} |
| 967 |
|
\end{table} |
| 968 |
|
|
| 880 |
– |
Although not discussed previously, group based cutoffs can be applied |
| 881 |
– |
to both the {\sc sp} and {\sc sf} methods. Use off a |
| 882 |
– |
switching function corrects for the discontinuities that arise when |
| 883 |
– |
atoms of a group exit the cutoff before the group's center of mass. |
| 884 |
– |
Though there are no significant benefit or drawbacks observed in |
| 885 |
– |
$\Delta E$ and vector magnitude results when doing this, there is a |
| 886 |
– |
measurable improvement in the vector angle results. Table |
| 887 |
– |
\ref{tab:groupAngle} shows the angular variance values obtained using |
| 888 |
– |
group based cutoffs and a switching function alongside the standard |
| 889 |
– |
results seen in figure \ref{fig:frcTrqAng} for comparison purposes. |
| 890 |
– |
The {\sc sp} shows much narrower angular distributions for |
| 891 |
– |
both the force and torque vectors when using an $\alpha$ of 0.2 |
| 892 |
– |
\AA$^{-1}$ or less, while {\sc sf} shows improvements in the |
| 893 |
– |
undamped and lightly damped cases. Thus, by calculating the |
| 894 |
– |
electrostatic interactions in terms of molecular pairs rather than |
| 895 |
– |
atomic pairs, the direction of the force and torque vectors are |
| 896 |
– |
determined more accurately. |
| 897 |
– |
|
| 969 |
|
One additional trend to recognize in table \ref{tab:groupAngle} is |
| 970 |
< |
that the $\sigma^2$ values for both {\sc sp} and |
| 971 |
< |
{\sc sf} converge as $\alpha$ increases, something that is easier |
| 972 |
< |
to see when using group based cutoffs. Looking back on figures |
| 973 |
< |
\ref{fig:delE}, \ref{fig:frcMag}, and \ref{fig:trqMag}, show this |
| 974 |
< |
behavior clearly at large $\alpha$ and cutoff values. The reason for |
| 975 |
< |
this is that the complimentary error function inserted into the |
| 976 |
< |
potential weakens the electrostatic interaction as $\alpha$ increases. |
| 977 |
< |
Thus, at larger values of $\alpha$, both the summation method types |
| 978 |
< |
progress toward non-interacting functions, so care is required in |
| 979 |
< |
choosing large damping functions lest one generate an undesirable loss |
| 980 |
< |
in the pair interaction. Kast \textit{et al.} developed a method for |
| 981 |
< |
choosing appropriate $\alpha$ values for these types of electrostatic |
| 982 |
< |
summation methods by fitting to $g(r)$ data, and their methods |
| 983 |
< |
indicate optimal values of 0.34, 0.25, and 0.16 \AA$^{-1}$ for cutoff |
| 984 |
< |
values of 9, 12, and 15 \AA\ respectively.\cite{Kast03} These appear |
| 985 |
< |
to be reasonable choices to obtain proper MC behavior |
| 986 |
< |
(Fig. \ref{fig:delE}); however, based on these findings, choices this |
| 987 |
< |
high would introduce error in the molecular torques, particularly for |
| 988 |
< |
the shorter cutoffs. Based on the above findings, empirical damping |
| 918 |
< |
up to 0.2 \AA$^{-1}$ proves to be beneficial, but damping is arguably |
| 919 |
< |
unnecessary when using the {\sc sf} method. |
| 970 |
> |
that the $\sigma^2$ values for both {\sc sp} and {\sc sf} converge as |
| 971 |
> |
$\alpha$ increases, something that is easier to see when using group |
| 972 |
> |
based cutoffs. The reason for this is that the complimentary error |
| 973 |
> |
function inserted into the potential weakens the electrostatic |
| 974 |
> |
interaction as $\alpha$ increases. Thus, at larger values of |
| 975 |
> |
$\alpha$, both summation methods progress toward non-interacting |
| 976 |
> |
functions, so care is required in choosing large damping functions |
| 977 |
> |
lest one generate an undesirable loss in the pair interaction. Kast |
| 978 |
> |
\textit{et al.} developed a method for choosing appropriate $\alpha$ |
| 979 |
> |
values for these types of electrostatic summation methods by fitting |
| 980 |
> |
to $g(r)$ data, and their methods indicate optimal values of 0.34, |
| 981 |
> |
0.25, and 0.16 \AA$^{-1}$ for cutoff values of 9, 12, and 15 \AA\ |
| 982 |
> |
respectively.\cite{Kast03} These appear to be reasonable choices to |
| 983 |
> |
obtain proper MC behavior (Fig. \ref{fig:delE}); however, based on |
| 984 |
> |
these findings, choices this high would introduce error in the |
| 985 |
> |
molecular torques, particularly for the shorter cutoffs. Based on the |
| 986 |
> |
above findings, empirical damping up to 0.2 \AA$^{-1}$ proves to be |
| 987 |
> |
beneficial, but damping may be unnecessary when using the {\sc sf} |
| 988 |
> |
method. |
| 989 |
|
|
| 990 |
|
\subsection{Short-Time Dynamics: Velocity Autocorrelation Functions of NaCl Crystals} |
| 991 |
|
|
| 992 |
< |
In the previous studies using a {\sc sf} variant of the damped |
| 993 |
< |
Wolf coulomb potential, the structure and dynamics of water were |
| 994 |
< |
investigated rather extensively.\cite{Zahn02,Kast03} Their results |
| 995 |
< |
indicated that the damped {\sc sf} method results in properties |
| 996 |
< |
very similar to those obtained when using the Ewald summation. |
| 997 |
< |
Considering the statistical results shown above, the good performance |
| 998 |
< |
of this method is not that surprising. Rather than consider the same |
| 999 |
< |
systems and simply recapitulate their results, we decided to look at |
| 1000 |
< |
the solid state dynamical behavior obtained using the best performing |
| 1001 |
< |
summation methods from the above results. |
| 992 |
> |
Zahn {\it et al.} investigated the structure and dynamics of water |
| 993 |
> |
using eqs. (\ref{eq:ZahnPot}) and |
| 994 |
> |
(\ref{eq:WolfForces}).\cite{Zahn02,Kast03} Their results indicated |
| 995 |
> |
that a method similar (but not identical with) the damped {\sc sf} |
| 996 |
> |
method resulted in properties very similar to those obtained when |
| 997 |
> |
using the Ewald summation. The properties they studied (pair |
| 998 |
> |
distribution functions, diffusion constants, and velocity and |
| 999 |
> |
orientational correlation functions) may not be particularly sensitive |
| 1000 |
> |
to the long-range and collective behavior that governs the |
| 1001 |
> |
low-frequency behavior in crystalline systems. |
| 1002 |
|
|
| 1003 |
+ |
We are using two separate measures to probe the effects of these |
| 1004 |
+ |
alternative electrostatic methods on the dynamics in crystalline |
| 1005 |
+ |
materials. For short- and intermediate-time dynamics, we are |
| 1006 |
+ |
computing the velocity autocorrelation function, and for long-time |
| 1007 |
+ |
and large length-scale collective motions, we are looking at the |
| 1008 |
+ |
low-frequency portion of the power spectrum. |
| 1009 |
+ |
|
| 1010 |
|
\begin{figure} |
| 1011 |
|
\centering |
| 1012 |
|
\includegraphics[width = \linewidth]{./vCorrPlot.pdf} |
| 1013 |
< |
\caption{Velocity auto-correlation functions of NaCl crystals at 1000 K while using SPME, {\sc sf} ($\alpha$ = 0.0, 0.1, \& 0.2), and {\sc sp} ($\alpha$ = 0.2). The inset is a magnification of the first trough. The times to first collision are nearly identical, but the differences can be seen in the peaks and troughs, where the undamped to weakly damped methods are stiffer than the moderately damped and SPME methods.} |
| 1013 |
> |
\caption{Velocity auto-correlation functions of NaCl crystals at |
| 1014 |
> |
1000 K using SPME, {\sc sf} ($\alpha$ = 0.0, 0.1, \& 0.2), and {\sc |
| 1015 |
> |
sp} ($\alpha$ = 0.2). The inset is a magnification of the area around |
| 1016 |
> |
the first minimum. The times to first collision are nearly identical, |
| 1017 |
> |
but differences can be seen in the peaks and troughs, where the |
| 1018 |
> |
undamped and weakly damped methods are stiffer than the moderately |
| 1019 |
> |
damped and SPME methods.} |
| 1020 |
|
\label{fig:vCorrPlot} |
| 1021 |
|
\end{figure} |
| 1022 |
|
|
| 1051 |
|
\begin{figure} |
| 1052 |
|
\centering |
| 1053 |
|
\includegraphics[width = \linewidth]{./spectraSquare.pdf} |
| 1054 |
< |
\caption{Power spectra obtained from the velocity auto-correlation functions of NaCl crystals at 1000 K while using SPME, {\sc sf} ($\alpha$ = 0, 0.1, \& 0.2), and {\sc sp} ($\alpha$ = 0.2). Apodization of the correlation functions via a cubic switching function between 40 and 50 ps was used to clear up the spectral noise resulting from data truncation, and had no noticeable effect on peak location or magnitude. The inset shows the frequency region below 100 cm$^{-1}$ to highlight where the spectra begin to differ.} |
| 1054 |
> |
\caption{Power spectra obtained from the velocity auto-correlation |
| 1055 |
> |
functions of NaCl crystals at 1000 K while using SPME, {\sc sf} |
| 1056 |
> |
($\alpha$ = 0, 0.1, \& 0.2), and {\sc sp} ($\alpha$ = 0.2). |
| 1057 |
> |
Apodization of the correlation functions via a cubic switching |
| 1058 |
> |
function between 40 and 50 ps was used to clear up the spectral noise |
| 1059 |
> |
resulting from data truncation, and had no noticeable effect on peak |
| 1060 |
> |
location or magnitude. The inset shows the frequency region below 100 |
| 1061 |
> |
cm$^{-1}$ to highlight where the spectra begin to differ.} |
| 1062 |
|
\label{fig:methodPS} |
| 1063 |
|
\end{figure} |
| 1064 |
|
|
| 1100 |
|
\begin{figure} |
| 1101 |
|
\centering |
| 1102 |
|
\includegraphics[width = \linewidth]{./comboSquare.pdf} |
| 1103 |
< |
\caption{Regions of spectra showing the low-frequency correlated motions for NaCl crystals at 1000 K using various electrostatic summation methods. The upper plot is a zoomed inset from figure \ref{fig:methodPS}. As the damping value for the {\sc sf} potential increases, the low-frequency peaks red-shift. The lower plot is of spectra when using SPME and a simple damped Coulombic sum with damping coefficients ($\alpha$) ranging from 0.15 to 0.3 \AA$^{-1}$. As $\alpha$ increases, the peaks are red-shifted toward and eventually beyond the values given by SPME. The larger $\alpha$ values weaken the real-space electrostatics, explaining this shift towards less strongly correlated motions in the crystal.} |
| 1103 |
> |
\caption{Regions of spectra showing the low-frequency correlated |
| 1104 |
> |
motions for NaCl crystals at 1000 K using various electrostatic |
| 1105 |
> |
summation methods. The upper plot is a zoomed inset from figure |
| 1106 |
> |
\ref{fig:methodPS}. As the damping value for the {\sc sf} potential |
| 1107 |
> |
increases, the low-frequency peaks red-shift. The lower plot is of |
| 1108 |
> |
spectra when using SPME and a simple damped Coulombic sum with damping |
| 1109 |
> |
coefficients ($\alpha$) ranging from 0.15 to 0.3 \AA$^{-1}$. As |
| 1110 |
> |
$\alpha$ increases, the peaks are red-shifted toward and eventually |
| 1111 |
> |
beyond the values given by SPME. The larger $\alpha$ values weaken |
| 1112 |
> |
the real-space electrostatics, explaining this shift towards less |
| 1113 |
> |
strongly correlated motions in the crystal.} |
| 1114 |
|
\label{fig:dampInc} |
| 1115 |
|
\end{figure} |
| 1116 |
|
|
