| 73 |
|
relevant transition temperatures and pressures for the model. |
| 74 |
|
|
| 75 |
|
In this paper, standard reference state methods were applied to known |
| 76 |
< |
crystalline water polymorphs in the low pressure regime. This work is |
| 77 |
< |
unique in that one of the crystal lattices was arrived at through |
| 78 |
< |
crystallization of a computationally efficient water model under |
| 79 |
< |
constant pressure and temperature conditions. Crystallization events |
| 80 |
< |
are interesting in and of themselves\cite{Matsumoto02,Yamada02}; |
| 81 |
< |
however, the crystal structure obtained in this case is different from |
| 82 |
< |
any previously observed ice polymorphs in experiment or |
| 83 |
< |
simulation.\cite{Fennell04} We have named this structure Ice-{\it i} |
| 84 |
< |
to indicate its origin in computational simulation. The unit cell of |
| 85 |
< |
Ice-{\it i} and an extruded variant named Ice-{\it i}$^\prime$ both |
| 86 |
< |
consist of eight water molecules that stack in rows of interlocking |
| 87 |
< |
water tetramers as illustrated in figures \ref{iCrystal}A and |
| 76 |
> |
crystalline water polymorphs to evaluate their free energy in the low |
| 77 |
> |
pressure regime. This work is unique in that one of the crystal |
| 78 |
> |
lattices was arrived at through crystallization of a computationally |
| 79 |
> |
efficient water model under constant pressure and temperature |
| 80 |
> |
conditions. Crystallization events are interesting in and of |
| 81 |
> |
themselves\cite{Matsumoto02,Yamada02}; however, the crystal structure |
| 82 |
> |
obtained in this case is different from any previously observed ice |
| 83 |
> |
polymorphs in experiment or simulation.\cite{Fennell04} We have named |
| 84 |
> |
this structure Ice-{\it i} to indicate its origin in computational |
| 85 |
> |
simulation. The unit cell of Ice-{\it i} and an extruded variant named |
| 86 |
> |
Ice-{\it i}$^\prime$ both consist of eight water molecules that stack |
| 87 |
> |
in rows of interlocking water tetramers as illustrated in figures |
| 88 |
> |
\ref{iCrystal}A and |
| 89 |
|
\ref{iCrystal}B. These tetramers make the crystal structure similar |
| 90 |
|
in appearance to a recent two-dimensional ice tessellation simulated |
| 91 |
|
on a silica surface.\cite{Yang04} As expected in an ice crystal |
| 283 |
|
Ewald correction flattens and narrows the sequences of free energies |
| 284 |
|
so much that they often overlap within error, indicating that other |
| 285 |
|
conditions, such as cell volume in microcanonical simulations, can |
| 286 |
< |
influence the chosen polymorph upon crystallization. All of these |
| 286 |
< |
results support the finding that the Ice-{\it i} polymorph is a stable |
| 287 |
< |
crystal structure that should be considered when studying the phase |
| 288 |
< |
behavior of water models. |
| 286 |
> |
influence the chosen polymorph upon crystallization. |
| 287 |
|
|
| 288 |
< |
Due to this relative stability of Ice-{\it i} in all of the |
| 289 |
< |
investigated simulation conditions, the question arises as to possible |
| 290 |
< |
experimental observation of this polymorph. The rather extensive past |
| 291 |
< |
and current experimental investigation of water in the low pressure |
| 292 |
< |
regime makes us hesitant to ascribe any relevance of this work outside |
| 293 |
< |
of the simulation community. It is for this reason that we chose a |
| 294 |
< |
name for this polymorph which involves an imaginary quantity. That |
| 295 |
< |
said, there are certain experimental conditions that would provide the |
| 296 |
< |
most ideal situation for possible observation. These include the |
| 297 |
< |
negative pressure or stretched solid regime, small clusters in vacuum |
| 288 |
> |
So what is the preferred solid polymorph for simulated water? The |
| 289 |
> |
answer appears to be dependent both on conditions and which model is |
| 290 |
> |
used. In the case of short cutoffs without a long-range interaction |
| 291 |
> |
correction, Ice-{\it i} and Ice-{\it i}$^\prime$ have the lowest free |
| 292 |
> |
energy of the studied polymorphs with all the models. Ideally, |
| 293 |
> |
crystallization of each model under constant pressure conditions, as |
| 294 |
> |
was done with SSD/E, would aid in the identification of their |
| 295 |
> |
respective preferred structures. This work, however, helps illustrate |
| 296 |
> |
how studies involving one specific model can lead to insight about |
| 297 |
> |
important behavior of others. In general, the above results support |
| 298 |
> |
the finding that the Ice-{\it i} polymorph is a stable crystal |
| 299 |
> |
structure that should be considered when studying the phase behavior |
| 300 |
> |
of water models. |
| 301 |
> |
|
| 302 |
> |
Finally, due to the stability of Ice-{\it i} in the investigated |
| 303 |
> |
simulation conditions, the question arises as to possible experimental |
| 304 |
> |
observation of this polymorph. The rather extensive past and current |
| 305 |
> |
experimental investigation of water in the low pressure regime makes |
| 306 |
> |
us hesitant to ascribe any relevance of this work outside of the |
| 307 |
> |
simulation community. It is for this reason that we chose a name for |
| 308 |
> |
this polymorph which involves an imaginary quantity. That said, there |
| 309 |
> |
are certain experimental conditions that would provide the most ideal |
| 310 |
> |
situation for possible observation. These include the negative |
| 311 |
> |
pressure or stretched solid regime, small clusters in vacuum |
| 312 |
|
deposition environments, and in clathrate structures involving small |
| 313 |
< |
non-polar molecules. Regardless of possible experimental observation, |
| 302 |
< |
the presence of these stable ice polymorphs has implications in the |
| 303 |
< |
understanding and depiction of phase changes involving the common |
| 304 |
< |
water models used in simulations. |
| 313 |
> |
non-polar molecules. |
| 314 |
|
|
| 315 |
|
\section{Acknowledgments} |
| 316 |
|
Support for this project was provided by the National Science |
