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# Line 3 | Line 3
3   \usepackage{endfloat}
4   \usepackage{amsmath}
5   \usepackage{amssymb}
6 + %\usepackage{ifsym}
7   \usepackage{epsf}
8   \usepackage{times}
9   \usepackage{mathptm}
# Line 23 | Line 24
24  
25   \begin{document}
26  
27 < \title{On the necessity of the Ewald Summation in molecular simulations: Alternatives to the accepted standard of cutoff policies}
27 > \title{Is the Ewald Summation necessary in typical molecular simulations: Alternatives to the accepted standard of cutoff policies}
28  
29   \author{Christopher J. Fennell and J. Daniel Gezelter \\
30   Department of Chemistry and Biochemistry\\
# Line 47 | Line 48 | In this paper, a variety of simulation situations were
48   \section{Introduction}
49  
50   In this paper, a variety of simulation situations were analyzed to determine the relative effectiveness of the adapted Wolf spherical truncation schemes at reproducing the results obtained using a smooth particle mesh Ewald (SPME) summation technique.  In addition to the Shifted-Potential and Shifted-Force adapted Wolf methods, both reaction field and uncorrected cutoff methods were included for comparison purposes.  The general usability of these methods in both Monte Carlo and Molecular Dynamics calculations was assessed through statistical analysis over the combined results from all of the following studied systems:
51 < \begin{list}{-}{}
51 > \begin{enumerate}
52   \item Liquid Water
53   \item Crystalline Water (Ice I$_\textrm{c}$)
54   \item NaCl Crystal
55   \item NaCl Melt
56 < \item 1 M Solution of NaCl in Water
57 < \item 10 M Solution of NaCl in Water
56 > \item Low Ionic Strength Solution of NaCl in Water
57 > \item High Ionic Strength Solution of NaCl in Water
58   \item 6 \AA\  Radius Sphere of Argon in Water
59 < \end{list}
59 > \end{enumerate}
60   Additional discussion on the results from the individual systems was also performed to identify limitations of the considered methods in specific systems.
61  
62   \section{Methods}
63  
64   In each of the simulated systems, 500 distinct configurations were generated, and the electrostatic summation methods were compared via sequential application on each of these fixed configurations.  The methods compared include SPME, the aforementioned Shifted Potential and Shifted Force methods - both with damping parameters ($\alpha$) of 0, 0.1, 0.2, and 0.3 \AA$^{-1}$, reaction field with an infinite dielectric constant, and an unmodified cutoff.  Group-based cutoffs with a fifth-order polynomial switching function were necessary for the reaction field simulations and were utilized in the SP, SF, and pure cutoff methods for comparison to the standard lack of group-based cutoffs with a hard truncation.  
65  
66 < Generation of the system configurations was dependent on the system type.  For the solid and liquid water configurations, configuration snapshots were taken at regular intervals from higher temperature 1000 SPC/E water molecule trajectories and individually equilibrated.  The solid and liquid NaCl systems consisted of 500 Na+ and 500 Cl- ions and were selected and equilibrated in the same fashion as the water systems.  For the 1 and 10 M NaCl solutions, 4 and 40 ions, respectively, were first solvated in a 1000 water molecule boxes.  Ion and water positions were then randomly swapped, and the resulting configurations were again individually equilibrated.  Finally, for the Argon/Water "charge void" systems, the identities of all the SPC/E waters within 6 \AA\ of the center of the equilibrated water configurations were converted to argon (Fig. \ref{argonSlice}).
66 > Generation of the system configurations was dependent on the system type.  For the solid and liquid water configurations, configuration snapshots were taken at regular intervals from higher temperature 1000 SPC/E water molecule trajectories and each equilibrated individually.  The solid and liquid NaCl systems consisted of 500 Na+ and 500 Cl- ions and were selected and equilibrated in the same fashion as the water systems.  For the low and high ionic strength NaCl solutions, 4 and 40 ions were first solvated in a 1000 water molecule boxes respectively.  Ion and water positions were then randomly swapped, and the resulting configurations were again equilibrated individually.  Finally, for the Argon/Water "charge void" systems, the identities of all the SPC/E waters within 6 \AA\ of the center of the equilibrated water configurations were converted to argon (Fig. \ref{argonSlice}).
67  
68   \begin{figure}
69   \centering
# Line 75 | Line 76 | In order to evaluate the performance of the adapted Wo
76  
77   \section{Results and Discussion}
78  
79 + \subsection{$\Delta E$ Comparison}
80   In order to evaluate the performance of the adapted Wolf Shifted Potential and Shifted Force electrostatic summation methods for Monte Carlo simulations, the energy differences between configurations need to be compared to the results using SPME.  Considering the SPME results to be the correct or desired behavior, ideal performance of a tested method is taken to be agreement between the energy differences calculated.  Linear least squares regression of the $\Delta E$ values between configurations using SPME against $\Delta E$ values using tested methods provides a quantitative comparison of this agreement.  Unitary results for both the correlation and correlation coefficient for these regressions indicate equivalent energetic results between the methods.  The correlation is the slope of the plotted data while the correlation coefficient ($R^2$) is a measure of the of the data scatter around the fitted line and gives an idea of the quality of the fit (Fig. \ref{linearFit}).
81  
82   \begin{figure}
# Line 84 | Line 86 | With 500 independent configurations, 124,750 $\Delta E
86   \label{linearFit}
87   \end{figure}
88  
89 < With 500 independent configurations, 124,750 $\Delta E$ data points are used in a regression of a single system.  Results and discussion for the individual analysis of each of the system types appear in the appendices of this paper.  To probe the applicability of each method in the general case, all the different system types were included in a single regression.  The results for this regression are shown in figure \ref{delE}.  
89 > With 500 independent configurations, 124,750 $\Delta E$ data points are used in a regression of a single system.  Results and discussion for the individual analysis of each of the system types appear in the supporting information.  To probe the applicability of each method in the general case, all the different system types were included in a single regression.  The results for this regression are shown in figure \ref{delE}.  
90  
91   \begin{figure}
92   \centering
# Line 93 | Line 95 | In figure \ref{delE}, it is readily apparent that it i
95   \label{delE}
96   \end{figure}
97  
98 < In figure \ref{delE}, it is readily apparent that it is unreasonable to expect realistic results using an unmodified cutoff.  This is not all that surprising since this results in large energy fluctuations as atoms move in and out of the cutoff radius.  These fluctuations can be alleviated to some degree by using group based cutoffs with a switching function.  The Group Switch Cutoff row doesn't show a significant improvement in this plot because the salt and salt solution systems contain non-neutral groups, see appendices \ref{app-water} and \ref{app-ice} for a comparison where all groups are neutral.  Correcting the resulting charged cutoff sphere is one of the purposes of the shifted potential proposed by Wolf \textit{et al.}, and this correction indeed improves the results as seen in the Shifted Potental rows.  While the undamped case of this method is a significant improvement over the pure cutoff, it still doesn't correlate that well with SPME.  Inclusion of potential damping improves the results, and using an $\alpha$ of 0.2 \AA $^{-1}$ shows an excellent correlation and quality of fit with the SPME results, particularly with a cutoff radius greater than 12 \AA .  Use of a larger damping parameter is more helpful for the shortest cutoff shown, but it has a detrimental effect on simulations with larger cutoffs.  This trend is repeated in the Shifted Force rows, where increasing damping results in progressively poorer correlation; however, damping looks to be unnecessary with this method.  Overall, the undamped case is the best performing set, as the correlation and quality of fits are consistently superior regardless of the cutoff distance.  This result is beneficial in that the undamped case is less computationally prohibitive do to the lack of complimentary error function calculation when performing the electrostatic pair interaction.  The reaction field results illustrates some of that method's limitations, primarily that it was developed for use in homogenous systems; although it does provide results that are an improvement over those from an unmodified cutoff.
98 > In figure \ref{delE}, it is apparent that it is unreasonable to expect realistic results using an unmodified cutoff.  This is not all that surprising since this results in large energy fluctuations as atoms move in and out of the cutoff radius.  These fluctuations can be alleviated to some degree by using group based cutoffs with a switching function.  The Group Switch Cutoff row doesn't show a significant improvement in this plot because the salt and salt solution systems contain non-neutral groups, see the accompanying supporting information for a comparison where all groups are neutral.  Correcting the resulting charged cutoff sphere is one of the purposes of the shifted potential proposed by Wolf \textit{et al.}, and this correction indeed improves the results as seen in the Shifted Potental rows.  While the undamped case of this method is a significant improvement over the pure cutoff, it still doesn't correlate that well with SPME.  Inclusion of potential damping improves the results, and using an $\alpha$ of 0.2 \AA $^{-1}$ shows an excellent correlation and quality of fit with the SPME results, particularly with a cutoff radius greater than 12 \AA .  Use of a larger damping parameter is more helpful for the shortest cutoff shown, but it has a detrimental effect on simulations with larger cutoffs.  This trend is repeated in the Shifted Force rows, where increasing damping results in progressively poorer correlation; however, damping looks to be unnecessary with this method.  Overall, the undamped case is the best performing set, as the correlation and quality of fits are consistently superior regardless of the cutoff distance.  This result is beneficial in that the undamped case is less computationally prohibitive do to the lack of complimentary error function calculation when performing the electrostatic pair interaction.  The reaction field results illustrates some of that method's limitations, primarily that it was developed for use in homogenous systems; although it does provide results that are an improvement over those from an unmodified cutoff.
99  
100 + \subsection{Force Magnitude Comparison}
101 +
102   While studying the energy differences provides insight into how comparable these methods are energetically, if we want to use these methods in Molecular Dynamics simulations, we also need to consider their effect on forces and torques.  Both the magnitude and the direction of the force and torque vectors of each of the bodies in the system can be compared to those observed while using SPME.  Analysis of the magnitude of these vectors can be performed in the manner described previously for comparing $\Delta E$ values, only instead of a single value between two system configurations, there is a value for each particle in each configuration.  For a system of 1000 water molecules and 40 ions, there are 1040 force vectors and 1000 torque vectors.  With 500 configurations, this results in excess of 500,000 data samples for each system type.  Figures \ref{frcMag} and \ref{trqMag} respectively show the force and torque vector magnitude results for the accumulated analysis over all the system types.
103  
104   \begin{figure}
# Line 106 | Line 110 | The results in figure \ref{frcMag} for the most part p
110  
111   The results in figure \ref{frcMag} for the most part parallel those seen in the previous look at the $\Delta E$ results.  The unmodified cutoff results are poor, but using group based cutoffs and a switching function provides a improvement much more significant than what was seen with $\Delta E$.  Looking at the Shifted Potential sets, the slope and R$^2$ improve with the use of damping to an optimal result of 0.2 \AA $^{-1}$ for the 12 and 15 \AA\ cutoffs.  Further increases in damping, while beneficial for simulations with a cutoff radius of 9 \AA\ , is detrimental to simulations with larger cutoff radii.  The undamped Shifted Force method gives forces in line with those obtained using SPME, and use of a damping function gives little to no gain.  The reaction field results are surprisingly good, considering the poor quality of the fits for the $\Delta E$ results.  There is still a considerable degree of scatter in the data, but it correlates well in general.
112  
113 + \subsection{Torque Magnitude Comparison}
114 +
115   \begin{figure}
116   \centering
117   \includegraphics[width=3.25in]{./trqMagplot.pdf}
# Line 115 | Line 121 | Having force and torque vectors with magnitudes that a
121  
122   The torque vector magnitude results in figure \ref{trqMag} are similar to those seen for the forces, but more clearly show the improved behavior with increasing cutoff radius.  Moderate damping is beneficial to the Shifted Potential and unnecessary with the Shifted Force method, and they also show that over-damping adversely effects all cutoff radii rather than showing an improvement for systems with short cutoffs.  The reaction field method performs well when calculating the torques, better than the Shifted Force method over this limited data set.
123  
124 + \subsection{Force and Torque Direction Comparison}
125 +
126   Having force and torque vectors with magnitudes that are well correlated to SPME is good, but if they are not pointing in the proper direction the results will be incorrect.  These vector directions were investigated through measurement of the angle formed between them and those from SPME.  The dot product of these unit vectors provides a theta value that is accumulated in a distribution function, weighted by the area on the unit sphere.  Narrow distributions of theta values indicates similar to identical results between the tested method and SPME.  To measure the narrowness of the resulting distributions, non-linear Gaussian fits were performed.
127  
128   \begin{figure}
# Line 133 | Line 141 | Both the force and torque $\sigma^2$ results from the
141   \label{frcTrqAng}
142   \end{figure}
143  
144 < Both the force and torque $\sigma^2$ results from the analysis of the total accumulated system data are tabulated in figure \ref{frcTrqAng}.  All of the sets, aside from the over-damped case show the improvement afforded by choosing a longer simulation cutoff.  Increasing the cutoff from 9 to 12 \AA\ typically results in a halving of $\sigma^2$, with a similar improvement going from 12 to 15 \AA .  The undamped Shifted Force, Group Based Cutoff, and Reaction Field methods all do equivalently well at capturing the direction of both the force and torque vectors.  Using damping improves the angular behavior significantly for the Shifted Potential and moderately for the Shifted Force methods.  Increasing the damping too far is destructive for both methods, particularly to the torque vectors.  Again it is important to recognize that the force vectors cover all particles in the systems, while torque vectors are only available for neutral molecular groups.  Damping appears to have a more beneficial non-neutral bodies, and this observation is investigated further in appendices \ref{app-melt}, \ref{app-salt}, \ref{app-sol1}, and \ref{app-sol10}.  
144 > Both the force and torque $\sigma^2$ results from the analysis of the total accumulated system data are tabulated in figure \ref{frcTrqAng}.  All of the sets, aside from the over-damped case show the improvement afforded by choosing a longer simulation cutoff.  Increasing the cutoff from 9 to 12 \AA\ typically results in a halving of $\sigma^2$, with a similar improvement going from 12 to 15 \AA .  The undamped Shifted Force, Group Based Cutoff, and Reaction Field methods all do equivalently well at capturing the direction of both the force and torque vectors.  Using damping improves the angular behavior significantly for the Shifted Potential and moderately for the Shifted Force methods.  Increasing the damping too far is destructive for both methods, particularly to the torque vectors.  Again it is important to recognize that the force vectors cover all particles in the systems, while torque vectors are only available for neutral molecular groups.  Damping appears to have a more beneficial non-neutral bodies, and this observation is investigated further in the accompanying supporting information.  
145  
146   \begin{table}[htbp]
147     \centering
148     \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$^2$.}  
149 <   \begin{tabular}{@{} ccrrrrrrrr @{}} % Column formatting, @{} suppresses leading/trailing space
149 >   \begin{tabular}{@{} ccrrrrrrrr @{}}
150        \\
151        \toprule
152        & & \multicolumn{4}{c}{Shifted Potential} & \multicolumn{4}{c}{Shifted Force} \\
153        \cmidrule(lr){3-6}
154        \cmidrule(l){7-10}
155 <            Cutoff Radius    & Groups & $\alpha = 0$ & $\alpha = 0.1$ & $\alpha = 0.2$ & $\alpha = 0.3$ & $\alpha = 0$ & $\alpha = 0.1$ & $\alpha = 0.2$ & $\alpha = 0.3$\\
155 >            $R_\textrm{c}$ & Groups & $\alpha = 0$ & $\alpha = 0.1$ & $\alpha = 0.2$ & $\alpha = 0.3$ & $\alpha = 0$ & $\alpha = 0.1$ & $\alpha = 0.2$ & $\alpha = 0.3$\\
156        \midrule
157 <      
158 <        9 \AA   & N & 29.545 & 12.003 & 5.489 & 0.610 & 2.323 & 2.321 & 0.429 & 0.603 \\
159 <                & \textbf{Y} & \textbf{2.486} & \textbf{2.160} & \textbf{0.667} & \textbf{0.608} & \textbf{1.768} & \textbf{1.766} & \textbf{0.676} & \textbf{0.609} \\
160 <        12 \AA  & N & 19.381 & 3.097 & 0.190 & 0.608 & 0.920 & 0.736 & 0.133 & 0.612 \\
161 <                & \textbf{Y} & \textbf{0.515} & \textbf{0.288} & \textbf{0.127} & \textbf{0.586} & \textbf{0.308} & \textbf{0.249} & \textbf{0.127} & \textbf{0.586} \\
162 <        15 \AA  & N & 12.700 & 1.196 & 0.123 & 0.601 & 0.339 & 0.160 & 0.123 & 0.601 \\
163 <                & \textbf{Y} & \textbf{0.228} & \textbf{0.099} & \textbf{0.121} & \textbf{0.598} & \textbf{0.144} & \textbf{0.090} & \textbf{0.121} & \textbf{0.598} \\      
157 >    
158 > 9 \AA   & N & 29.545 & 12.003 & 5.489 & 0.610 & 2.323 & 2.321 & 0.429 & 0.603 \\
159 >        & \textbf{Y} & \textbf{2.486} & \textbf{2.160} & \textbf{0.667} & \textbf{0.608} & \textbf{1.768} & \textbf{1.766} & \textbf{0.676} & \textbf{0.609} \\
160 > 12 \AA  & N & 19.381 & 3.097 & 0.190 & 0.608 & 0.920 & 0.736 & 0.133 & 0.612 \\
161 >        & \textbf{Y} & \textbf{0.515} & \textbf{0.288} & \textbf{0.127} & \textbf{0.586} & \textbf{0.308} & \textbf{0.249} & \textbf{0.127} & \textbf{0.586} \\
162 > 15 \AA  & N & 12.700 & 1.196 & 0.123 & 0.601 & 0.339 & 0.160 & 0.123 & 0.601 \\
163 >        & \textbf{Y} & \textbf{0.228} & \textbf{0.099} & \textbf{0.121} & \textbf{0.598} & \textbf{0.144} & \textbf{0.090} & \textbf{0.121} & \textbf{0.598} \\      
164  
165        \midrule
166        
167 <        9 \AA   & N & 262.716 & 116.585 & 5.234 & 5.103 & 2.392 & 2.350 & 1.770 & 5.122 \\
168 <                & \textbf{Y} & \textbf{2.115} & \textbf{1.914} & \textbf{1.878} & \textbf{5.142} & \textbf{2.076} & \textbf{2.039} & \textbf{1.972} & \textbf{5.146} \\
169 <        12 \AA  & N & 129.576 & 25.560 & 1.369 & 5.080 & 0.913 & 0.790 & 1.362 & 5.124 \\
170 <                & \textbf{Y} & \textbf{0.810} & \textbf{0.685} & \textbf{1.352} & \textbf{5.082} & \textbf{0.765} & \textbf{0.714} & \textbf{1.360} & \textbf{5.082} \\
171 <        15 \AA  & N & 87.275 & 4.473 & 1.271 & 5.000 & 0.372 & 0.312 & 1.271 & 5.000 \\
172 <                & \textbf{Y} & \textbf{0.282} & \textbf{0.294} & \textbf{1.272} & \textbf{4.999} & \textbf{0.324} & \textbf{0.318} & \textbf{1.272} & \textbf{4.999} \\
167 > 9 \AA   & N & 262.716 & 116.585 & 5.234 & 5.103 & 2.392 & 2.350 & 1.770 & 5.122 \\
168 >        & \textbf{Y} & \textbf{2.115} & \textbf{1.914} & \textbf{1.878} & \textbf{5.142} & \textbf{2.076} & \textbf{2.039} & \textbf{1.972} & \textbf{5.146} \\
169 > 12 \AA  & N & 129.576 & 25.560 & 1.369 & 5.080 & 0.913 & 0.790 & 1.362 & 5.124 \\
170 >        & \textbf{Y} & \textbf{0.810} & \textbf{0.685} & \textbf{1.352} & \textbf{5.082} & \textbf{0.765} & \textbf{0.714} & \textbf{1.360} & \textbf{5.082} \\
171 > 15 \AA  & N & 87.275 & 4.473 & 1.271 & 5.000 & 0.372 & 0.312 & 1.271 & 5.000 \\
172 >        & \textbf{Y} & \textbf{0.282} & \textbf{0.294} & \textbf{1.272} & \textbf{4.999} & \textbf{0.324} & \textbf{0.318} & \textbf{1.272} & \textbf{4.999} \\
173  
174        \bottomrule
175     \end{tabular}
# Line 175 | Line 183 | One additional trend to recognize in table \ref{groupA
183   \section{Conclusions}
184  
185   \section{Acknowledgments}
178
179 \appendix
180
181 \section{\label{app-water}Liquid Water}
182
183 \begin{table}[htbp]
184   \centering
185   \caption{Regression results for the liquid water system. Tabulated results include $\Delta E$ values (top set), force vector magnitudes (middle set) and torque vector magnitudes (bottom set).  PC = Pure Cutoff, SP = Shifted Potential, SF = Shifted Force, GSC = Group Switched Cutoff, and RF = Reaction Field (where $\varepsilon \approx \infty$).}  
186   \begin{tabular}{@{} ccrrrrrr @{}} % Column formatting, @{} suppresses leading/trailing space
187      \\
188      \toprule
189      & & \multicolumn{2}{c}{9 \AA} & \multicolumn{2}{c}{12 \AA} & \multicolumn{2}{c}{15 \AA}\\
190      \cmidrule(lr){3-4}
191      \cmidrule(lr){5-6}
192      \cmidrule(l){7-8}
193            Method & $\alpha$ & slope & $R^2$ & slope & $R^2$ & slope & $R^2$ \\
194            \midrule
195 PC  &     & 3.046 & 0.002 & -3.018 & 0.002 & 4.719 & 0.005 \\
196 SP  & 0.0 & 1.035 & 0.218 & 0.908 & 0.313 & 1.037 & 0.470 \\
197    & 0.1 & 1.021 & 0.387 & 0.965 & 0.752 & 1.006 & 0.947 \\
198    & 0.2 & 0.997 & 0.962 & 1.001 & 0.994 & 0.994 & 0.996 \\
199    & 0.3 & 0.984 & 0.980 & 0.997 & 0.985 & 0.982 & 0.987 \\
200 SF  & 0.0 & 0.977 & 0.974 & 0.996 & 0.992 & 0.991 & 0.997 \\
201    & 0.1 & 0.983 & 0.974 & 1.001 & 0.994 & 0.996 & 0.998 \\
202    & 0.2 & 0.992 & 0.989 & 1.001 & 0.995 & 0.994 & 0.996 \\
203    & 0.3 & 0.984 & 0.980 & 0.996 & 0.985 & 0.982 & 0.987 \\
204 GSC &     & 0.918 & 0.862 & 0.852 & 0.756 & 0.801 & 0.700 \\
205 RF  &     & 0.971 & 0.958 & 0.975 & 0.987 & 0.959 & 0.983 \\                              
206
207            \midrule
208
209 PC  &     & -1.647 & 0.000 & -0.127 & 0.000 & -0.979 & 0.000 \\
210 SP  & 0.0 & 0.735 & 0.368 & 0.813 & 0.537 & 0.865 & 0.659 \\
211    & 0.1 & 0.850 & 0.612 & 0.956 & 0.887 & 0.992 & 0.979 \\
212    & 0.2 & 0.996 & 0.989 & 1.000 & 1.000 & 1.000 & 1.000 \\
213    & 0.3 & 0.996 & 0.998 & 0.997 & 0.998 & 0.996 & 0.998 \\
214 SF  & 0.0 & 0.998 & 0.995 & 1.000 & 0.999 & 1.000 & 0.999 \\
215    & 0.1 & 0.998 & 0.995 & 1.000 & 0.999 & 1.000 & 1.000 \\
216    & 0.2 & 0.999 & 0.998 & 1.000 & 1.000 & 1.000 & 1.000 \\
217    & 0.3 & 0.996 & 0.998 & 0.997 & 0.998 & 0.996 & 0.998 \\
218 GSC &     & 0.998 & 0.995 & 1.000 & 0.999 & 1.000 & 1.000 \\
219 RF  &     & 0.999 & 0.995 & 1.000 & 0.999 & 1.000 & 1.000 \\          
220
221            \midrule
222
223 PC  &     & 2.387 & 0.000 & 0.183 & 0.000 & 1.282 & 0.000 \\
224 SP  & 0.0 & 0.847 & 0.543 & 0.904 & 0.694 & 0.935 & 0.786 \\
225    & 0.1 & 0.922 & 0.749 & 0.980 & 0.934 & 0.996 & 0.988 \\
226    & 0.2 & 0.987 & 0.985 & 0.989 & 0.992 & 0.990 & 0.993 \\
227    & 0.3 & 0.965 & 0.973 & 0.967 & 0.975 & 0.967 & 0.976 \\
228 SF  & 0.0 & 0.978 & 0.990 & 0.988 & 0.997 & 0.993 & 0.999 \\
229    & 0.1 & 0.983 & 0.991 & 0.993 & 0.997 & 0.997 & 0.999 \\
230    & 0.2 & 0.986 & 0.989 & 0.989 & 0.992 & 0.990 & 0.993 \\
231    & 0.3 & 0.965 & 0.973 & 0.967 & 0.975 & 0.967 & 0.976 \\
232 GSC &     & 0.995 & 0.981 & 0.999 & 0.991 & 1.001 & 0.994 \\
233 RF  &     & 0.993 & 0.989 & 0.998 & 0.996 & 1.000 & 0.999 \\
234      \bottomrule
235   \end{tabular}
236   \label{spceTabTMag}
237 \end{table}
238
239 \begin{table}[htbp]
240   \centering
241   \caption{Variance results from Gaussian fits to angular distributions of the force and torque vectors in the liquid water system.  PC = Pure Cutoff, SP = Shifted Potential, SF = Shifted Force, GSC = Group Switched Cutoff, RF = Reaction Field (where $\varepsilon \approx \infty$), GSSP = Group Switched Shifted Potential, and GSSF = Group Switched Shifted Force.}  
242   \begin{tabular}{@{} ccrrrrrr @{}} % Column formatting, @{} suppresses leading/trailing space
243      \\
244      \toprule
245      & & \multicolumn{3}{c}{Force $\sigma^2$} & \multicolumn{3}{c}{Torque $\sigma^2$} \\
246      \cmidrule(lr){3-5}
247      \cmidrule(l){6-8}
248            Method & $\alpha$ & 9 \AA & 12 \AA & 15 \AA & 9 \AA & 12 \AA & 15 \AA \\
249            \midrule
250 PC  &     & 783.759 & 481.353 & 332.677 & 248.674 & 144.382 & 98.535 \\
251 SP  & 0.0 & 659.440 & 380.699 & 250.002 & 235.151 & 134.661 & 88.135 \\
252    & 0.1 & 293.849 & 67.772 & 11.609 & 105.090 & 23.813 & 4.369 \\
253    & 0.2 & 5.975 & 0.136 & 0.094 & 5.553 & 1.784 & 1.536 \\
254    & 0.3 & 0.725 & 0.707 & 0.693 & 7.293 & 6.933 & 6.748 \\
255 SF  & 0.0 & 2.238 & 0.713 & 0.292 & 3.290 & 1.090 & 0.416 \\
256    & 0.1 & 2.238 & 0.524 & 0.115 & 3.184 & 0.945 & 0.326 \\
257    & 0.2 & 0.374 & 0.102 & 0.094 & 2.598 & 1.755 & 1.537 \\
258    & 0.3 & 0.721 & 0.707 & 0.693 & 7.322 & 6.933 & 6.748 \\
259 GSC &     & 2.431 & 0.614 & 0.274 & 5.135 & 2.133 & 1.339 \\
260 RF  &     & 2.091 & 0.403 & 0.113 & 3.583 & 1.071 & 0.399 \\      
261                        \midrule
262 GSSP  & 0.0 & 2.431 & 0.614 & 0.274 & 5.135 & 2.133 & 1.339 \\
263      & 0.1 & 1.879 & 0.291 & 0.057 & 3.983 & 1.117 & 0.370 \\
264      & 0.2 & 0.443 & 0.103 & 0.093 & 2.821 & 1.794 & 1.532 \\
265      & 0.3 & 0.728 & 0.694 & 0.692 & 7.387 & 6.942 & 6.748 \\
266 GSSF  & 0.0 & 1.298 & 0.270 & 0.083 & 3.098 & 0.992 & 0.375 \\
267      & 0.1 & 1.296 & 0.210 & 0.044 & 3.055 & 0.922 & 0.330 \\
268      & 0.2 & 0.433 & 0.104 & 0.093 & 2.895 & 1.797 & 1.532 \\
269      & 0.3 & 0.728 & 0.694 & 0.692 & 7.410 & 6.942 & 6.748 \\
270      \bottomrule
271   \end{tabular}
272   \label{spceTabAng}
273 \end{table}
274
275 \section{\label{app-ice}Solid Water: Ice I$_\textrm{c}$}
276
277 \begin{table}[htbp]
278   \centering
279   \caption{Regression results for the ice I$_\textrm{c}$ system. Tabulated results include $\Delta E$ values (top set), force vector magnitudes (middle set) and torque vector magnitudes (bottom set).  PC = Pure Cutoff, SP = Shifted Potential, SF = Shifted Force, GSC = Group Switched Cutoff, and RF = Reaction Field (where $\varepsilon \approx \infty$).}    
280   \begin{tabular}{@{} ccrrrrrr @{}} % Column formatting, @{} suppresses leading/trailing space
281      \\
282      \toprule
283      & & \multicolumn{2}{c}{9 \AA} & \multicolumn{2}{c}{12 \AA} & \multicolumn{2}{c}{15 \AA}\\
284      \cmidrule(lr){3-4}
285      \cmidrule(lr){5-6}
286      \cmidrule(l){7-8}
287            Method & $\alpha$ & slope & $R^2$ & slope & $R^2$ & slope & $R^2$ \\
288            \midrule
289 PC  &     & 19.897 & 0.047 & -29.214 & 0.048 & -3.771 & 0.001 \\
290 SP  & 0.0 & -0.014 & 0.000 & 2.135 & 0.347 & 0.457 & 0.045 \\
291    & 0.1 & 0.321 & 0.017 & 1.490 & 0.584 & 0.886 & 0.796 \\
292    & 0.2 & 0.896 & 0.872 & 1.011 & 0.998 & 0.997 & 0.999 \\
293    & 0.3 & 0.983 & 0.997 & 0.992 & 0.997 & 0.991 & 0.997 \\
294 SF  & 0.0 & 0.943 & 0.979 & 1.048 & 0.978 & 0.995 & 0.999 \\
295    & 0.1 & 0.948 & 0.979 & 1.044 & 0.983 & 1.000 & 0.999 \\
296    & 0.2 & 0.982 & 0.997 & 0.969 & 0.960 & 0.997 & 0.999 \\
297    & 0.3 & 0.985 & 0.997 & 0.961 & 0.961 & 0.991 & 0.997 \\
298 GSC &     & 0.983 & 0.985 & 0.966 & 0.994 & 1.003 & 0.999 \\
299 RF  &     & 0.924 & 0.944 & 0.990 & 0.996 & 0.991 & 0.998 \\
300            \midrule
301 PC  &     & -4.375 & 0.000 & 6.781 & 0.000 & -3.369 & 0.000 \\
302 SP  & 0.0 & 0.515 & 0.164 & 0.856 & 0.426 & 0.743 & 0.478 \\
303    & 0.1 & 0.696 & 0.405 & 0.977 & 0.817 & 0.974 & 0.964 \\
304    & 0.2 & 0.981 & 0.980 & 1.001 & 1.000 & 1.000 & 1.000 \\
305    & 0.3 & 0.996 & 0.998 & 0.997 & 0.999 & 0.997 & 0.999 \\
306 SF  & 0.0 & 0.991 & 0.995 & 1.003 & 0.998 & 0.999 & 1.000 \\
307    & 0.1 & 0.992 & 0.995 & 1.003 & 0.998 & 1.000 & 1.000 \\
308    & 0.2 & 0.998 & 0.998 & 0.981 & 0.962 & 1.000 & 1.000 \\
309    & 0.3 & 0.996 & 0.998 & 0.976 & 0.957 & 0.997 & 0.999 \\
310 GSC &     & 0.997 & 0.996 & 0.998 & 0.999 & 1.000 & 1.000 \\
311 RF  &     & 0.988 & 0.989 & 1.000 & 0.999 & 1.000 & 1.000 \\
312            \midrule
313 PC  &     & -6.367 & 0.000 & -3.552 & 0.000 & -3.447 & 0.000 \\
314 SP  & 0.0 & 0.643 & 0.409 & 0.833 & 0.607 & 0.961 & 0.805 \\
315    & 0.1 & 0.791 & 0.683 & 0.957 & 0.914 & 1.000 & 0.989 \\
316    & 0.2 & 0.974 & 0.991 & 0.993 & 0.998 & 0.993 & 0.998 \\
317    & 0.3 & 0.976 & 0.992 & 0.977 & 0.992 & 0.977 & 0.992 \\
318 SF  & 0.0 & 0.979 & 0.997 & 0.992 & 0.999 & 0.994 & 1.000 \\
319    & 0.1 & 0.984 & 0.997 & 0.996 & 0.999 & 0.998 & 1.000 \\
320    & 0.2 & 0.991 & 0.997 & 0.974 & 0.958 & 0.993 & 0.998 \\
321    & 0.3 & 0.977 & 0.992 & 0.956 & 0.948 & 0.977 & 0.992 \\
322 GSC &     & 0.999 & 0.997 & 0.996 & 0.999 & 1.002 & 1.000 \\
323 RF  &     & 0.994 & 0.997 & 0.997 & 0.999 & 1.000 & 1.000 \\
324      \bottomrule
325   \end{tabular}
326   \label{iceTab}
327 \end{table}
328
329 \begin{table}[htbp]
330   \centering
331   \caption{Variance results from Gaussian fits to angular distributions of the force and torque vectors in the ice I$_\textrm{c}$ system.  PC = Pure Cutoff, SP = Shifted Potential, SF = Shifted Force, GSC = Group Switched Cutoff, RF = Reaction Field (where $\varepsilon \approx \infty$), GSSP = Group Switched Shifted Potential, and GSSF = Group Switched Shifted Force.}    
332   \begin{tabular}{@{} ccrrrrrr @{}} % Column formatting, @{} suppresses leading/trailing space
333      \\
334      \toprule
335      & & \multicolumn{3}{c}{Force $\sigma^2$} & \multicolumn{3}{c}{Torque $\sigma^2$} \\
336      \cmidrule(lr){3-5}
337      \cmidrule(l){6-8}
338            Method & $\alpha$ & 9 \AA & 12 \AA & 15 \AA & 9 \AA & 12 \AA & 15 \AA \\
339            \midrule
340 PC  &     & 2128.921 & 603.197 & 715.579 & 329.056 & 221.397 & 81.042 \\
341 SP  & 0.0 & 1429.341 & 470.320 & 447.557 & 301.678 & 197.437 & 73.840 \\
342    & 0.1 & 590.008 & 107.510 & 18.883 & 118.201 & 32.472 & 3.599 \\
343    & 0.2 & 10.057 & 0.105 & 0.038 & 2.875 & 0.572 & 0.518 \\
344    & 0.3 & 0.245 & 0.260 & 0.262 & 2.365 & 2.396 & 2.327 \\
345 SF  & 0.0 & 1.745 & 1.161 & 0.212 & 1.135 & 0.426 & 0.155 \\
346    & 0.1 & 1.721 & 0.868 & 0.082 & 1.118 & 0.358 & 0.118 \\
347    & 0.2 & 0.201 & 0.040 & 0.038 & 0.786 & 0.555 & 0.518 \\
348    & 0.3 & 0.241 & 0.260 & 0.262 & 2.368 & 2.400 & 2.327 \\
349 GSC &     & 1.483 & 0.261 & 0.099 & 0.926 & 0.295 & 0.095 \\
350 RF  &     & 2.887 & 0.217 & 0.107 & 1.006 & 0.281 & 0.085 \\
351                        \midrule
352 GSSP  & 0.0 & 1.483 & 0.261 & 0.099 & 0.926 & 0.295 & 0.095 \\
353      & 0.1 & 1.341 & 0.123 & 0.037 & 0.835 & 0.234 & 0.085 \\
354      & 0.2 & 0.558 & 0.040 & 0.037 & 0.823 & 0.557 & 0.519 \\
355      & 0.3 & 0.250 & 0.251 & 0.259 & 2.387 & 2.395 & 2.328 \\
356 GSSF  & 0.0 & 2.124 & 0.132 & 0.069 & 0.919 & 0.263 & 0.099 \\
357      & 0.1 & 2.165 & 0.101 & 0.035 & 0.895 & 0.244 & 0.096 \\
358      & 0.2 & 0.706 & 0.040 & 0.037 & 0.870 & 0.559 & 0.519 \\
359      & 0.3 & 0.251 & 0.251 & 0.259 & 2.387 & 2.395 & 2.328 \\
360      \bottomrule
361   \end{tabular}
362   \label{iceTabAng}
363 \end{table}
364
365 \section{\label{app-melt}NaCl Melt}
366
367 \begin{table}[htbp]
368   \centering
369   \caption{Regression results for the molten NaCl system. Tabulated results include $\Delta E$ values (top set) and force vector magnitudes (bottom set).  PC = Pure Cutoff, SP = Shifted Potential, and SF = Shifted Force.}  
370   \begin{tabular}{@{} ccrrrrrr @{}} % Column formatting, @{} suppresses leading/trailing space
371      \\
372      \toprule
373      & & \multicolumn{2}{c}{9 \AA} & \multicolumn{2}{c}{12 \AA} & \multicolumn{2}{c}{15 \AA}\\
374      \cmidrule(lr){3-4}
375      \cmidrule(lr){5-6}
376      \cmidrule(l){7-8}
377            Method & $\alpha$ & slope & $R^2$ & slope & $R^2$ & slope & $R^2$ \\
378            \midrule
379 PC  &     & -0.008 & 0.000 & -0.049 & 0.005 & -0.136 & 0.020 \\
380 SP  & 0.0 & 0.937 & 0.996 & 0.880 & 0.995 & 0.971 & 0.999 \\
381    & 0.1 & 1.004 & 0.999 & 0.958 & 1.000 & 0.928 & 0.994 \\
382    & 0.2 & 0.960 & 1.000 & 0.813 & 0.996 & 0.811 & 0.954 \\
383    & 0.3 & 0.671 & 0.994 & 0.439 & 0.929 & 0.535 & 0.831 \\
384 SF  & 0.0 & 1.001 & 1.000 & 0.949 & 1.000 & 1.008 & 1.000 \\
385    & 0.1 & 1.025 & 1.000 & 0.960 & 1.000 & 0.929 & 0.994 \\
386    & 0.2 & 0.966 & 1.000 & 0.813 & 0.996 & 0.811 & 0.954 \\
387    & 0.3 & 0.671 & 0.994 & 0.439 & 0.929 & 0.535 & 0.831 \\
388            \midrule
389 PC  &     & 1.103 & 0.000 & 0.989 & 0.000 & 0.802 & 0.000 \\
390 SP  & 0.0 & 0.976 & 0.983 & 1.001 & 0.991 & 0.985 & 0.995 \\
391    & 0.1 & 0.996 & 0.997 & 0.997 & 0.998 & 0.996 & 0.996 \\
392    & 0.2 & 0.993 & 0.996 & 0.985 & 0.988 & 0.986 & 0.981 \\
393    & 0.3 & 0.956 & 0.956 & 0.940 & 0.912 & 0.948 & 0.929 \\
394 SF  & 0.0 & 0.997 & 0.998 & 0.995 & 0.999 & 0.999 & 1.000 \\
395    & 0.1 & 1.001 & 0.997 & 0.997 & 0.999 & 0.996 & 0.996 \\
396    & 0.2 & 0.994 & 0.996 & 0.985 & 0.988 & 0.986 & 0.981 \\
397    & 0.3 & 0.956 & 0.956 & 0.940 & 0.912 & 0.948 & 0.929 \\
398      \bottomrule
399   \end{tabular}
400   \label{meltTab}
401 \end{table}
402
403 \begin{table}[htbp]
404   \centering
405   \caption{Variance results from Gaussian fits to angular distributions of the force vectors in the molten NaCl system.  PC = Pure Cutoff, SP = Shifted Potential, and SF = Shifted Force.}    
406   \begin{tabular}{@{} ccrrrrrr @{}} % Column formatting, @{} suppresses leading/trailing space
407      \\
408      \toprule
409      & & \multicolumn{3}{c}{Force $\sigma^2$} \\
410      \cmidrule(lr){3-5}
411      \cmidrule(l){6-8}
412            Method & $\alpha$ & 9 \AA & 12 \AA & 15 \AA \\
413            \midrule
414 PC  &     & 13.294 & 8.035 & 5.366 \\
415 SP  & 0.0 & 13.316 & 8.037 & 5.385 \\
416    & 0.1 & 5.705 & 1.391 & 0.360 \\
417    & 0.2 & 2.415 & 7.534 & 13.927 \\
418    & 0.3 & 23.769 & 67.306 & 57.252 \\
419 SF  & 0.0 & 1.693 & 0.603 & 0.256 \\
420    & 0.1 & 1.687 & 0.653 & 0.272 \\
421    & 0.2 & 2.598 & 7.523 & 13.930 \\
422    & 0.3 & 23.734 & 67.305 & 57.252 \\
423      \bottomrule
424   \end{tabular}
425   \label{meltTabAng}
426 \end{table}
427
428 \section{\label{app-salt}NaCl Crystal}
429
430 \begin{table}[htbp]
431   \centering
432   \caption{Regression results for the crystalline NaCl system. Tabulated results include $\Delta E$ values (top set) and force vector magnitudes (bottom set).  PC = Pure Cutoff, SP = Shifted Potential, and SF = Shifted Force.}    
433   \begin{tabular}{@{} ccrrrrrr @{}} % Column formatting, @{} suppresses leading/trailing space
434      \\
435      \toprule
436      & & \multicolumn{2}{c}{9 \AA} & \multicolumn{2}{c}{12 \AA} & \multicolumn{2}{c}{15 \AA}\\
437      \cmidrule(lr){3-4}
438      \cmidrule(lr){5-6}
439      \cmidrule(l){7-8}
440            Method & $\alpha$ & slope & $R^2$ & slope & $R^2$ & slope & $R^2$ \\
441            \midrule
442 PC  &     & -20.241 & 0.228 & -20.248 & 0.229 & -20.239 & 0.228 \\
443 SP  & 0.0 & 1.039 & 0.733 & 2.037 & 0.565 & 1.225 & 0.743 \\
444    & 0.1 & 1.049 & 0.865 & 1.424 & 0.784 & 1.029 & 0.980 \\
445    & 0.2 & 0.982 & 0.976 & 0.969 & 0.980 & 0.960 & 0.980 \\
446    & 0.3 & 0.873 & 0.944 & 0.872 & 0.945 & 0.872 & 0.945 \\
447 SF  & 0.0 & 1.041 & 0.967 & 0.994 & 0.989 & 0.957 & 0.993 \\
448    & 0.1 & 1.050 & 0.968 & 0.996 & 0.991 & 0.972 & 0.995 \\
449    & 0.2 & 0.982 & 0.975 & 0.959 & 0.980 & 0.960 & 0.980 \\
450    & 0.3 & 0.873 & 0.944 & 0.872 & 0.945 & 0.872 & 0.944 \\
451            \midrule
452 PC  &     & 0.795 & 0.000 & 0.792 & 0.000 & 0.793 & 0.000 \\
453 SP  & 0.0 & 0.916 & 0.829 & 1.086 & 0.791 & 1.010 & 0.936 \\
454    & 0.1 & 0.958 & 0.917 & 1.049 & 0.943 & 1.001 & 0.995 \\
455    & 0.2 & 0.981 & 0.981 & 0.982 & 0.984 & 0.981 & 0.984 \\
456    & 0.3 & 0.950 & 0.952 & 0.950 & 0.953 & 0.950 & 0.953 \\
457 SF  & 0.0 & 1.002 & 0.983 & 0.997 & 0.994 & 0.991 & 0.997 \\
458    & 0.1 & 1.003 & 0.984 & 0.996 & 0.995 & 0.993 & 0.997 \\
459    & 0.2 & 0.983 & 0.980 & 0.981 & 0.984 & 0.981 & 0.984 \\
460    & 0.3 & 0.950 & 0.952 & 0.950 & 0.953 & 0.950 & 0.953 \\
461      \bottomrule
462   \end{tabular}
463   \label{saltTab}
464 \end{table}
186  
466 \begin{table}[htbp]
467   \centering
468   \caption{Variance results from Gaussian fits to angular distributions of the force vectors in the crystalline NaCl system.  PC = Pure Cutoff, SP = Shifted Potential, SF = Shifted Force, GSC = Group Switched Cutoff, and RF = Reaction Field (where $\varepsilon \approx \infty$).}        
469   \begin{tabular}{@{} ccrrrrrr @{}} % Column formatting, @{} suppresses leading/trailing space
470      \\
471      \toprule
472      & & \multicolumn{3}{c}{Force $\sigma^2$} \\
473      \cmidrule(lr){3-5}
474      \cmidrule(l){6-8}
475            Method & $\alpha$ & 9 \AA & 12 \AA & 15 \AA \\
476            \midrule
477 PC  &     & 111.945 & 111.824 & 111.866 \\
478 SP  & 0.0 & 112.414 & 152.215 & 38.087 \\
479    & 0.1 & 52.361 & 42.574 & 2.819 \\
480    & 0.2 & 10.847 & 9.709 & 9.686 \\
481    & 0.3 & 31.128 & 31.104 & 31.029 \\
482 SF  & 0.0 & 10.025 & 3.555 & 1.648 \\
483    & 0.1 & 9.462 & 3.303 & 1.721 \\
484    & 0.2 & 11.454 & 9.813 & 9.701 \\
485    & 0.3 & 31.120 & 31.105 & 31.029 \\
486      \bottomrule
487   \end{tabular}
488   \label{saltTabAng}
489 \end{table}
490
491 \section{\label{app-sol1}1M NaCl Solution}
492
493 \begin{table}[htbp]
494   \centering
495   \caption{Regression results for the 1M NaCl solution system. Tabulated results include $\Delta E$ values (top set), force vector magnitudes (middle set) and torque vector magnitudes (bottom set).  PC = Pure Cutoff, SP = Shifted Potential, SF = Shifted Force, GSC = Group Switched Cutoff, RF = Reaction Field (where $\varepsilon \approx \infty$), GSSP = Group Switched Shifted Potential, and GSSF = Group Switched Shifted Force.}
496   \begin{tabular}{@{} ccrrrrrr @{}} % Column formatting, @{} suppresses leading/trailing space
497      \\
498      \toprule
499      & & \multicolumn{2}{c}{9 \AA} & \multicolumn{2}{c}{12 \AA} & \multicolumn{2}{c}{15 \AA}\\
500      \cmidrule(lr){3-4}
501      \cmidrule(lr){5-6}
502      \cmidrule(l){7-8}
503            Method & $\alpha$ & slope & $R^2$ & slope & $R^2$ & slope & $R^2$ \\
504            \midrule
505 PC  &     & 0.247 & 0.000 & -1.103 & 0.001 & 5.480 & 0.015 \\
506 SP  & 0.0 & 0.935 & 0.388 & 0.984 & 0.541 & 1.010 & 0.685 \\
507    & 0.1 & 0.951 & 0.603 & 0.993 & 0.875 & 1.001 & 0.979 \\
508    & 0.2 & 0.969 & 0.968 & 0.996 & 0.997 & 0.994 & 0.997 \\
509    & 0.3 & 0.955 & 0.966 & 0.984 & 0.992 & 0.978 & 0.991 \\
510 SF  & 0.0 & 0.963 & 0.971 & 0.989 & 0.996 & 0.991 & 0.998 \\
511    & 0.1 & 0.970 & 0.971 & 0.995 & 0.997 & 0.997 & 0.999 \\
512    & 0.2 & 0.972 & 0.975 & 0.996 & 0.997 & 0.994 & 0.997 \\
513    & 0.3 & 0.955 & 0.966 & 0.984 & 0.992 & 0.978 & 0.991 \\
514 GSC &     & 0.964 & 0.731 & 0.984 & 0.704 & 1.005 & 0.770 \\
515 RF  &     & 0.968 & 0.605 & 0.974 & 0.541 & 1.014 & 0.614 \\
516            \midrule
517 PC  &     & 1.354 & 0.000 & -1.190 & 0.000 & -0.314 & 0.000 \\
518 SP  & 0.0 & 0.720 & 0.338 & 0.808 & 0.523 & 0.860 & 0.643 \\
519    & 0.1 & 0.839 & 0.583 & 0.955 & 0.882 & 0.992 & 0.978 \\
520    & 0.2 & 0.995 & 0.987 & 0.999 & 1.000 & 0.999 & 1.000 \\
521    & 0.3 & 0.995 & 0.996 & 0.996 & 0.998 & 0.996 & 0.998 \\
522 SF  & 0.0 & 0.998 & 0.994 & 1.000 & 0.998 & 1.000 & 0.999 \\
523    & 0.1 & 0.997 & 0.994 & 1.000 & 0.999 & 1.000 & 1.000 \\
524    & 0.2 & 0.999 & 0.998 & 0.999 & 1.000 & 0.999 & 1.000 \\
525    & 0.3 & 0.995 & 0.996 & 0.996 & 0.998 & 0.996 & 0.998 \\
526 GSC &     & 0.995 & 0.990 & 0.998 & 0.997 & 0.998 & 0.996 \\
527 RF  &     & 0.998 & 0.993 & 0.999 & 0.998 & 0.999 & 0.996 \\
528            \midrule
529 PC  &     & 2.437 & 0.000 & -1.872 & 0.000 & 2.138 & 0.000 \\
530 SP  & 0.0 & 0.838 & 0.525 & 0.901 & 0.686 & 0.932 & 0.779 \\
531    & 0.1 & 0.914 & 0.733 & 0.979 & 0.932 & 0.995 & 0.987 \\
532    & 0.2 & 0.977 & 0.969 & 0.988 & 0.990 & 0.989 & 0.990 \\
533    & 0.3 & 0.952 & 0.950 & 0.964 & 0.971 & 0.965 & 0.970 \\
534 SF  & 0.0 & 0.969 & 0.977 & 0.987 & 0.996 & 0.993 & 0.998 \\
535    & 0.1 & 0.975 & 0.978 & 0.993 & 0.996 & 0.997 & 0.998 \\
536    & 0.2 & 0.976 & 0.973 & 0.988 & 0.990 & 0.989 & 0.990 \\
537    & 0.3 & 0.952 & 0.950 & 0.964 & 0.971 & 0.965 & 0.970 \\
538 GSC &     & 0.980 & 0.959 & 0.990 & 0.983 & 0.992 & 0.989 \\
539 RF  &     & 0.984 & 0.975 & 0.996 & 0.995 & 0.998 & 0.998 \\
540      \bottomrule
541   \end{tabular}
542   \label{sol1Tab}
543 \end{table}
544
545 \begin{table}[htbp]
546   \centering
547   \caption{Variance results from Gaussian fits to angular distributions of the force and torque vectors in the 1M NaCl solution system.  PC = Pure Cutoff, SP = Shifted Potential, SF = Shifted Force, GSC = Group Switched Cutoff, RF = Reaction Field (where $\varepsilon \approx \infty$), GSSP = Group Switched Shifted Potential, and GSSF = Group Switched Shifted Force.}      
548   \begin{tabular}{@{} ccrrrrrr @{}} % Column formatting, @{} suppresses leading/trailing space
549      \\
550      \toprule
551      & & \multicolumn{3}{c}{Force $\sigma^2$} & \multicolumn{3}{c}{Torque $\sigma^2$} \\
552      \cmidrule(lr){3-5}
553      \cmidrule(l){6-8}
554            Method & $\alpha$ & 9 \AA & 12 \AA & 15 \AA & 9 \AA & 12 \AA & 15 \AA \\
555            \midrule
556 PC  &     & 882.863 & 510.435 & 344.201 & 277.691 & 154.231 & 100.131 \\
557 SP  & 0.0 & 732.569 & 405.704 & 257.756 & 261.445 & 142.245 & 91.497 \\
558    & 0.1 & 329.031 & 70.746 & 12.014 & 118.496 & 25.218 & 4.711 \\
559    & 0.2 & 6.772 & 0.153 & 0.118 & 9.780 & 2.101 & 2.102 \\
560    & 0.3 & 0.951 & 0.774 & 0.784 & 12.108 & 7.673 & 7.851 \\
561 SF  & 0.0 & 2.555 & 0.762 & 0.313 & 6.590 & 1.328 & 0.558 \\
562    & 0.1 & 2.561 & 0.560 & 0.123 & 6.464 & 1.162 & 0.457 \\
563    & 0.2 & 0.501 & 0.118 & 0.118 & 5.698 & 2.074 & 2.099 \\
564    & 0.3 & 0.943 & 0.774 & 0.784 & 12.118 & 7.674 & 7.851 \\
565 GSC &     & 2.915 & 0.643 & 0.261 & 9.576 & 3.133 & 1.812 \\
566 RF  &     & 2.415 & 0.452 & 0.130 & 6.915 & 1.423 & 0.507 \\
567                        \midrule
568 GSSP  & 0.0 & 2.915 & 0.643 & 0.261 & 9.576 & 3.133 & 1.812 \\
569      & 0.1 & 2.251 & 0.324 & 0.064 & 7.628 & 1.639 & 0.497 \\
570      & 0.2 & 0.590 & 0.118 & 0.116 & 6.080 & 2.096 & 2.103 \\
571      & 0.3 & 0.953 & 0.759 & 0.780 & 12.347 & 7.683 & 7.849 \\
572 GSSF  & 0.0 & 1.541 & 0.301 & 0.096 & 6.407 & 1.316 & 0.496 \\
573      & 0.1 & 1.541 & 0.237 & 0.050 & 6.356 & 1.202 & 0.457 \\
574      & 0.2 & 0.568 & 0.118 & 0.116 & 6.166 & 2.105 & 2.105 \\
575      & 0.3 & 0.954 & 0.759 & 0.780 & 12.337 & 7.684 & 7.849 \\
576      \bottomrule
577   \end{tabular}
578   \label{sol1TabAng}
579 \end{table}
580
581 \section{\label{app-sol10}10M NaCl Solution}
582
583 \begin{table}[htbp]
584   \centering
585   \caption{Regression results for the 10M NaCl solution system. Tabulated results include $\Delta E$ values (top set), force vector magnitudes (middle set) and torque vector magnitudes (bottom set).  PC = Pure Cutoff, SP = Shifted Potential, SF = Shifted Force, GSC = Group Switched Cutoff, and RF = Reaction Field (where $\varepsilon \approx \infty$).}      
586   \begin{tabular}{@{} ccrrrrrr @{}} % Column formatting, @{} suppresses leading/trailing space
587      \\
588      \toprule
589      & & \multicolumn{2}{c}{9 \AA} & \multicolumn{2}{c}{12 \AA} & \multicolumn{2}{c}{15 \AA}\\
590      \cmidrule(lr){3-4}
591      \cmidrule(lr){5-6}
592      \cmidrule(l){7-8}
593            Method & $\alpha$ & slope & $R^2$ & slope & $R^2$ & slope & $R^2$ \\
594            \midrule
595 PC  &     & -0.081 & 0.000 & 0.945 & 0.001 & 0.073 & 0.000 \\
596 SP  & 0.0 & 0.978 & 0.469 & 0.996 & 0.672 & 0.975 & 0.668 \\
597    & 0.1 & 0.944 & 0.645 & 0.997 & 0.886 & 0.991 & 0.978 \\
598    & 0.2 & 0.873 & 0.896 & 0.985 & 0.993 & 0.980 & 0.993 \\
599    & 0.3 & 0.831 & 0.860 & 0.960 & 0.979 & 0.955 & 0.977 \\
600 SF  & 0.0 & 0.858 & 0.905 & 0.985 & 0.970 & 0.990 & 0.998 \\
601    & 0.1 & 0.865 & 0.907 & 0.992 & 0.974 & 0.994 & 0.999 \\
602    & 0.2 & 0.862 & 0.894 & 0.985 & 0.993 & 0.980 & 0.993 \\
603    & 0.3 & 0.831 & 0.859 & 0.960 & 0.979 & 0.955 & 0.977 \\
604 GSC &     & 1.985 & 0.152 & 0.760 & 0.031 & 1.106 & 0.062 \\
605 RF  &     & 2.414 & 0.116 & 0.813 & 0.017 & 1.434 & 0.047 \\
606            \midrule
607 PC  &     & -7.028 & 0.000 & -9.364 & 0.000 & 0.925 & 0.865 \\
608 SP  & 0.0 & 0.701 & 0.319 & 0.909 & 0.773 & 0.861 & 0.665 \\
609    & 0.1 & 0.824 & 0.565 & 0.970 & 0.930 & 0.990 & 0.979 \\
610    & 0.2 & 0.988 & 0.981 & 0.995 & 0.998 & 0.991 & 0.998 \\
611    & 0.3 & 0.983 & 0.985 & 0.985 & 0.991 & 0.978 & 0.990 \\
612 SF  & 0.0 & 0.993 & 0.988 & 0.992 & 0.984 & 0.998 & 0.999 \\
613    & 0.1 & 0.993 & 0.989 & 0.993 & 0.986 & 0.998 & 1.000 \\
614    & 0.2 & 0.993 & 0.992 & 0.995 & 0.998 & 0.991 & 0.998 \\
615    & 0.3 & 0.983 & 0.985 & 0.985 & 0.991 & 0.978 & 0.990 \\
616 GSC &     & 0.964 & 0.897 & 0.970 & 0.917 & 0.925 & 0.865 \\
617 RF  &     & 0.994 & 0.864 & 0.988 & 0.865 & 0.980 & 0.784 \\
618            \midrule
619 PC  &     & -2.212 & 0.000 & -0.588 & 0.000 & 0.953 & 0.925 \\
620 SP  & 0.0 & 0.800 & 0.479 & 0.930 & 0.804 & 0.924 & 0.759 \\
621    & 0.1 & 0.883 & 0.694 & 0.976 & 0.942 & 0.993 & 0.986 \\
622    & 0.2 & 0.952 & 0.943 & 0.980 & 0.984 & 0.980 & 0.983 \\
623    & 0.3 & 0.914 & 0.909 & 0.943 & 0.948 & 0.944 & 0.946 \\
624 SF  & 0.0 & 0.945 & 0.953 & 0.980 & 0.984 & 0.991 & 0.998 \\
625    & 0.1 & 0.951 & 0.954 & 0.987 & 0.986 & 0.995 & 0.998 \\
626    & 0.2 & 0.951 & 0.946 & 0.980 & 0.984 & 0.980 & 0.983 \\
627    & 0.3 & 0.914 & 0.908 & 0.943 & 0.948 & 0.944 & 0.946 \\
628 GSC &     & 0.882 & 0.818 & 0.939 & 0.902 & 0.953 & 0.925 \\
629 RF  &     & 0.949 & 0.939 & 0.988 & 0.988 & 0.992 & 0.993 \\
630      \bottomrule
631   \end{tabular}
632   \label{sol10Tab}
633 \end{table}
634
635 \begin{table}[htbp]
636   \centering
637   \caption{Variance results from Gaussian fits to angular distributions of the force and torque vectors in the 10M NaCl solution system.  PC = Pure Cutoff, SP = Shifted Potential, SF = Shifted Force, GSC = Group Switched Cutoff, RF = Reaction Field (where $\varepsilon \approx \infty$), GSSP = Group Switched Shifted Potential, and GSSF = Group Switched Shifted Force.}      
638   \begin{tabular}{@{} ccrrrrrr @{}} % Column formatting, @{} suppresses leading/trailing space
639      \\
640      \toprule
641      & & \multicolumn{3}{c}{Force $\sigma^2$} & \multicolumn{3}{c}{Torque $\sigma^2$} \\
642      \cmidrule(lr){3-5}
643      \cmidrule(l){6-8}
644            Method & $\alpha$ & 9 \AA & 12 \AA & 15 \AA & 9 \AA & 12 \AA & 15 \AA \\
645            \midrule
646 PC  &     & 957.784 & 513.373 & 2.260 & 340.043 & 179.443 & 13.079 \\
647 SP  & 0.0 & 786.244 & 139.985 & 259.289 & 311.519 & 90.280 & 105.187 \\
648    & 0.1 & 354.697 & 38.614 & 12.274 & 144.531 & 23.787 & 5.401 \\
649    & 0.2 & 7.674 & 0.363 & 0.215 & 16.655 & 3.601 & 3.634 \\
650    & 0.3 & 1.745 & 1.456 & 1.449 & 23.669 & 14.376 & 14.240 \\
651 SF  & 0.0 & 3.282 & 8.567 & 0.369 & 11.904 & 6.589 & 0.717 \\
652    & 0.1 & 3.263 & 7.479 & 0.142 & 11.634 & 5.750 & 0.591 \\
653    & 0.2 & 0.686 & 0.324 & 0.215 & 10.809 & 3.580 & 3.635 \\
654    & 0.3 & 1.749 & 1.456 & 1.449 & 23.635 & 14.375 & 14.240 \\
655 GSC &     & 6.181 & 2.904 & 2.263 & 44.349 & 19.442 & 12.873 \\
656 RF  &     & 3.891 & 0.847 & 0.323 & 18.628 & 3.995 & 2.072 \\
657                        \midrule
658 GSSP  & 0.0 & 6.197 & 2.929 & 2.290 & 44.441 & 19.442 & 12.873 \\
659      & 0.1 & 4.688 & 1.064 & 0.260 & 31.208 & 6.967 & 2.303 \\
660      & 0.2 & 1.021 & 0.218 & 0.213 & 14.425 & 3.629 & 3.649 \\
661      & 0.3 & 1.752 & 1.454 & 1.451 & 23.540 & 14.390 & 14.245 \\
662 GSSF  & 0.0 & 2.494 & 0.546 & 0.217 & 16.391 & 3.230 & 1.613 \\
663      & 0.1 & 2.448 & 0.429 & 0.106 & 16.390 & 2.827 & 1.159 \\
664      & 0.2 & 0.899 & 0.214 & 0.213 & 13.542 & 3.583 & 3.645 \\
665      & 0.3 & 1.752 & 1.454 & 1.451 & 23.587 & 14.390 & 14.245 \\
666      \bottomrule
667   \end{tabular}
668   \label{sol10TabAng}
669 \end{table}
670
671 \section{\label{app-argon}Argon Sphere in Water}
672
673 \begin{table}[htbp]
674   \centering
675   \caption{Regression results for the 6 \AA\ argon sphere in liquid water system. Tabulated results include $\Delta E$ values (top set), force vector magnitudes (middle set) and torque vector magnitudes (bottom set).  PC = Pure Cutoff, SP = Shifted Potential, SF = Shifted Force, GSC = Group Switched Cutoff, and RF = Reaction Field (where $\varepsilon \approx \infty$).}    
676   \begin{tabular}{@{} ccrrrrrr @{}} % Column formatting, @{} suppresses leading/trailing space
677      \\
678      \toprule
679      & & \multicolumn{2}{c}{9 \AA} & \multicolumn{2}{c}{12 \AA} & \multicolumn{2}{c}{15 \AA}\\
680      \cmidrule(lr){3-4}
681      \cmidrule(lr){5-6}
682      \cmidrule(l){7-8}
683            Method & $\alpha$ & slope & $R^2$ & slope & $R^2$ & slope & $R^2$ \\
684            \midrule
685 PC  &     & 2.320 & 0.008 & -0.650 & 0.001 & 3.848 & 0.029 \\
686 SP  & 0.0 & 1.053 & 0.711 & 0.977 & 0.820 & 0.974 & 0.882 \\
687    & 0.1 & 1.032 & 0.846 & 0.989 & 0.965 & 0.992 & 0.994 \\
688    & 0.2 & 0.993 & 0.995 & 0.982 & 0.998 & 0.986 & 0.998 \\
689    & 0.3 & 0.968 & 0.995 & 0.954 & 0.992 & 0.961 & 0.994 \\
690 SF  & 0.0 & 0.982 & 0.996 & 0.992 & 0.999 & 0.993 & 1.000 \\
691    & 0.1 & 0.987 & 0.996 & 0.996 & 0.999 & 0.997 & 1.000 \\
692    & 0.2 & 0.989 & 0.998 & 0.984 & 0.998 & 0.989 & 0.998 \\
693    & 0.3 & 0.971 & 0.995 & 0.957 & 0.992 & 0.965 & 0.994 \\
694 GSC &     & 1.002 & 0.983 & 0.992 & 0.973 & 0.996 & 0.971 \\
695 RF  &     & 0.998 & 0.995 & 0.999 & 0.998 & 0.998 & 0.998 \\
696            \midrule
697 PC  &     & -36.559 & 0.002 & -44.917 & 0.004 & -52.945 & 0.006 \\
698 SP  & 0.0 & 0.890 & 0.786 & 0.927 & 0.867 & 0.949 & 0.909 \\
699    & 0.1 & 0.942 & 0.895 & 0.984 & 0.974 & 0.997 & 0.995 \\
700    & 0.2 & 0.999 & 0.997 & 1.000 & 1.000 & 1.000 & 1.000 \\
701    & 0.3 & 1.001 & 0.999 & 1.001 & 1.000 & 1.001 & 1.000 \\
702 SF  & 0.0 & 1.000 & 0.999 & 1.000 & 1.000 & 1.000 & 1.000 \\
703    & 0.1 & 1.000 & 0.999 & 1.000 & 1.000 & 1.000 & 1.000 \\
704    & 0.2 & 1.000 & 1.000 & 1.000 & 1.000 & 1.000 & 1.000 \\
705    & 0.3 & 1.001 & 0.999 & 1.001 & 1.000 & 1.001 & 1.000 \\
706 GSC &     & 0.999 & 0.999 & 1.000 & 1.000 & 1.000 & 1.000 \\
707 RF  &     & 0.999 & 0.999 & 1.000 & 1.000 & 1.000 & 1.000 \\
708            \midrule
709 PC  &     & 1.984 & 0.000 & 0.012 & 0.000 & 1.357 & 0.000 \\
710 SP  & 0.0 & 0.850 & 0.552 & 0.907 & 0.703 & 0.938 & 0.793 \\
711    & 0.1 & 0.924 & 0.755 & 0.980 & 0.936 & 0.995 & 0.988 \\
712    & 0.2 & 0.985 & 0.983 & 0.986 & 0.988 & 0.987 & 0.988 \\
713    & 0.3 & 0.961 & 0.966 & 0.959 & 0.964 & 0.960 & 0.966 \\
714 SF  & 0.0 & 0.977 & 0.989 & 0.987 & 0.995 & 0.992 & 0.998 \\
715    & 0.1 & 0.982 & 0.989 & 0.992 & 0.996 & 0.997 & 0.998 \\
716    & 0.2 & 0.984 & 0.987 & 0.986 & 0.987 & 0.987 & 0.988 \\
717    & 0.3 & 0.961 & 0.966 & 0.959 & 0.964 & 0.960 & 0.966 \\
718 GSC &     & 0.995 & 0.981 & 0.999 & 0.990 & 1.000 & 0.993 \\
719 RF  &     & 0.993 & 0.988 & 0.997 & 0.995 & 0.999 & 0.998 \\
720      \bottomrule
721   \end{tabular}
722   \label{argonTab}
723 \end{table}
724
725 \begin{table}[htbp]
726   \centering
727   \caption{Variance results from Gaussian fits to angular distributions of the force and torque vectors in the 6 \AA\ sphere of argon in liquid water system.  PC = Pure Cutoff, SP = Shifted Potential, SF = Shifted Force, GSC = Group Switched Cutoff, RF = Reaction Field (where $\varepsilon \approx \infty$), GSSP = Group Switched Shifted Potential, and GSSF = Group Switched Shifted Force.}
728   \begin{tabular}{@{} ccrrrrrr @{}} % Column formatting, @{} suppresses leading/trailing space
729      \\
730      \toprule
731      & & \multicolumn{3}{c}{Force $\sigma^2$} & \multicolumn{3}{c}{Torque $\sigma^2$} \\
732      \cmidrule(lr){3-5}
733      \cmidrule(l){6-8}
734            Method & $\alpha$ & 9 \AA & 12 \AA & 15 \AA & 9 \AA & 12 \AA & 15 \AA \\
735            \midrule
736 PC  &     & 568.025 & 265.993 & 195.099 & 246.626 & 138.600 & 91.654 \\
737 SP  & 0.0 & 504.578 & 251.694 & 179.932 & 231.568 & 131.444 & 85.119 \\
738    & 0.1 & 224.886 & 49.746 & 9.346 & 104.482 & 23.683 & 4.480 \\
739    & 0.2 & 4.889 & 0.197 & 0.155 & 6.029 & 2.507 & 2.269 \\
740    & 0.3 & 0.817 & 0.833 & 0.812 & 8.286 & 8.436 & 8.135 \\
741 SF  & 0.0 & 1.924 & 0.675 & 0.304 & 3.658 & 1.448 & 0.600 \\
742    & 0.1 & 1.937 & 0.515 & 0.143 & 3.565 & 1.308 & 0.546 \\
743    & 0.2 & 0.407 & 0.166 & 0.156 & 3.086 & 2.501 & 2.274 \\
744    & 0.3 & 0.815 & 0.833 & 0.812 & 8.330 & 8.437 & 8.135 \\
745 GSC &     & 2.098 & 0.584 & 0.284 & 5.391 & 2.414 & 1.501 \\
746 RF  &     & 1.822 & 0.408 & 0.142 & 3.799 & 1.362 & 0.550 \\
747                        \midrule
748 GSSP  & 0.0 & 2.098 & 0.584 & 0.284 & 5.391 & 2.414 & 1.501 \\
749      & 0.1 & 1.652 & 0.309 & 0.087 & 4.197 & 1.401 & 0.590 \\
750      & 0.2 & 0.465 & 0.165 & 0.153 & 3.323 & 2.529 & 2.273 \\
751      & 0.3 & 0.813 & 0.825 & 0.816 & 8.316 & 8.447 & 8.132 \\
752 GSSF  & 0.0 & 1.173 & 0.292 & 0.113 & 3.452 & 1.347 & 0.583 \\
753      & 0.1 & 1.166 & 0.240 & 0.076 & 3.381 & 1.281 & 0.575 \\
754      & 0.2 & 0.459 & 0.165 & 0.153 & 3.430 & 2.542 & 2.273 \\
755      & 0.3 & 0.814 & 0.825 & 0.816 & 8.325 & 8.447 & 8.132 \\
756      \bottomrule
757   \end{tabular}
758   \label{argonTabAng}
759 \end{table}
760
187   \newpage
188  
189   \bibliographystyle{achemso}

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