| 241 |
|
risk of spontaneous crystallization. However, this risk changes when |
| 242 |
|
applying a longer cutoff. |
| 243 |
|
|
| 244 |
+ |
Increasing the cutoff radius in simulations of the more |
| 245 |
+ |
computationally efficient water models was done in order to evaluate |
| 246 |
+ |
the trend in free energy values when moving to systems that do not |
| 247 |
+ |
involve potential truncation. As seen in Fig. \ref{incCutoff}, the |
| 248 |
+ |
free energy of all the ice polymorphs show a substantial dependence on |
| 249 |
+ |
cutoff radius. In general, there is a narrowing of the free energy |
| 250 |
+ |
differences while moving to greater cutoff radius. This trend is much |
| 251 |
+ |
more subtle in the case of SSD/RF, indicating that the free energies |
| 252 |
+ |
calculated with a reaction field present provide a more accurate |
| 253 |
+ |
picture of the free energy landscape in the absence of potential |
| 254 |
+ |
truncation. |
| 255 |
+ |
|
| 256 |
+ |
To further study the changes resulting to the inclusion of a |
| 257 |
+ |
long-range interaction correction, the effect of an Ewald summation |
| 258 |
+ |
was estimated by applying the potential energy difference do to its |
| 259 |
+ |
inclusion in systems in the presence and absence of the |
| 260 |
+ |
correction. This was accomplished by calculation of the potential |
| 261 |
+ |
energy of identical crystals with and without PME using TINKER. The |
| 262 |
+ |
free energies for the investigated polymorphs using the TIP3P and |
| 263 |
+ |
SPC/E water models are shown in Table \ref{pmeShift}. TIP4P and TIP5P |
| 264 |
+ |
are not fully supported in TINKER, so the results for these models |
| 265 |
+ |
could not be estimated. The same trend pointed out through increase of |
| 266 |
+ |
cutoff radius is observed in these results. Ice-{\it i} is the |
| 267 |
+ |
preferred polymorph at ambient conditions for both the TIP3P and SPC/E |
| 268 |
+ |
water models; however, there is a narrowing of the free energy |
| 269 |
+ |
differences between the various solid forms. In the case of SPC/E this |
| 270 |
+ |
narrowing is significant enough that it becomes less clear cut that |
| 271 |
+ |
Ice-{\it i} is the most stable polymorph, and is possibly metastable |
| 272 |
+ |
with respect to ice B and possibly ice $I_c$. However, these results |
| 273 |
+ |
do not significantly alter the finding that the Ice-{\it i} polymorph |
| 274 |
+ |
is a stable crystal structure that should be considered when studying |
| 275 |
+ |
the phase behavior of water models. |
| 276 |
|
|
| 277 |
+ |
\begin{table*} |
| 278 |
+ |
\begin{minipage}{\linewidth} |
| 279 |
+ |
\renewcommand{\thefootnote}{\thempfootnote} |
| 280 |
+ |
\begin{center} |
| 281 |
+ |
\caption{The free energy of the studied ice polymorphs after applying the energy difference attributed to the inclusion of the PME long-range interaction correction. Units are kcal/mol.} |
| 282 |
+ |
\begin{tabular}{ l c c c c } |
| 283 |
+ |
\hline \\[-7mm] |
| 284 |
+ |
\ \ Water Model \ \ & \ \ \ \ \ $I_h$ \ \ & \ \ \ \ \ $I_c$ \ \ & \ \quad \ \ \ \ B \ \ & \ \ \ \ \ Ice-{\it i} \ \ \\ |
| 285 |
+ |
\hline \\[-3mm] |
| 286 |
+ |
\ \ TIP3P & \ \ -11.53 & \ \ -11.24 & \ \ -11.51 & \ \ -11.67\\ |
| 287 |
+ |
\ \ SPC/E & \ \ -12.77 & \ \ -12.92 & \ \ -12.96 & \ \ -13.02\\ |
| 288 |
+ |
\end{tabular} |
| 289 |
+ |
\label{pmeShift} |
| 290 |
+ |
\end{center} |
| 291 |
+ |
\end{minipage} |
| 292 |
+ |
\end{table*} |
| 293 |
|
|
| 294 |
|
\section{Conclusions} |
| 295 |
|
|
