388 |
|
conditions, such as the density in fixed-volume simulations, can |
389 |
|
influence the polymorph expressed upon crystallization. |
390 |
|
|
391 |
< |
So what is the preferred solid polymorph for simulated water? The |
392 |
< |
answer appears to be dependent both on the conditions and the model |
393 |
< |
used. In the case of short cutoffs without a long-range interaction |
394 |
< |
correction, Ice-{\it i} and Ice-{\it i}$^\prime$ have the lowest free |
395 |
< |
energy of the studied polymorphs with all the models. Ideally, |
396 |
< |
crystallization of each model under constant pressure conditions, as |
397 |
< |
was done with SSD/E, would aid in the identification of their |
398 |
< |
respective preferred structures. This work, however, helps illustrate |
399 |
< |
how studies involving one specific model can lead to insight about |
400 |
< |
important behavior of others. In general, the above results support |
401 |
< |
the finding that the Ice-{\it i} polymorph is a stable crystal |
402 |
< |
structure that should be considered when studying the phase behavior |
403 |
< |
of water models. |
391 |
> |
\section{Conclusions} |
392 |
|
|
393 |
+ |
In this report, thermodynamic integration was used to determine the |
394 |
+ |
absolute free energies of several ice polymorphs. Of the studied |
395 |
+ |
crystal forms, Ice-{\it i} was observed to be the stable crystalline |
396 |
+ |
state for {\it all} the water models when using a 9.0 \AA\ |
397 |
+ |
intermolecular interaction cutoff. Through investigation of possible |
398 |
+ |
interaction truncation methods, the free energy was shown to be |
399 |
+ |
partially dependent on simulation conditions; however, Ice-{\it i} was |
400 |
+ |
still observered to be a stable polymorph of the studied water models. |
401 |
+ |
|
402 |
+ |
So what is the preferred solid polymorph for simulated water? As |
403 |
+ |
indicated above, the answer appears to be dependent both on the |
404 |
+ |
conditions and the model used. In the case of short cutoffs without a |
405 |
+ |
long-range interaction correction, Ice-{\it i} and Ice-{\it |
406 |
+ |
i}$^\prime$ have the lowest free energy of the studied polymorphs with |
407 |
+ |
all the models. Ideally, crystallization of each model under constant |
408 |
+ |
pressure conditions, as was done with SSD/E, would aid in the |
409 |
+ |
identification of their respective preferred structures. This work, |
410 |
+ |
however, helps illustrate how studies involving one specific model can |
411 |
+ |
lead to insight about important behavior of others. In general, the |
412 |
+ |
above results support the finding that the Ice-{\it i} polymorph is a |
413 |
+ |
stable crystal structure that should be considered when studying the |
414 |
+ |
phase behavior of water models. |
415 |
+ |
|
416 |
|
We also note that none of the water models used in this study are |
417 |
|
polarizable or flexible models. It is entirely possible that the |
418 |
|
polarizability of real water makes Ice-{\it i} substantially less |