| 484 |
|
|
| 485 |
|
In this work, thermodynamic integration was used to determine the |
| 486 |
|
absolute free energies of several ice polymorphs. The new polymorph, |
| 487 |
< |
Ice-{\it i} was observed to be the stable crystalline state for {\it |
| 487 |
> |
Ice-$i$ was observed to be the stable crystalline state for {\it |
| 488 |
|
all} the water models when using a 9.0~\AA\ cutoff. However, the free |
| 489 |
|
energy partially depends on simulation conditions (particularly on the |
| 490 |
< |
choice of long range correction method). Regardless, Ice-{\it i} was |
| 490 |
> |
choice of long range correction method). Regardless, Ice-$i$ was |
| 491 |
|
still observed to be a stable polymorph for all of the studied water |
| 492 |
|
models. |
| 493 |
|
|
| 494 |
|
So what is the preferred solid polymorph for simulated water? As |
| 495 |
|
indicated above, the answer appears to be dependent both on the |
| 496 |
|
conditions and the model used. In the case of short cutoffs without a |
| 497 |
< |
long-range interaction correction, Ice-{\it i} and Ice-$i^\prime$ have |
| 497 |
> |
long-range interaction correction, Ice-$i$ and Ice-$i^\prime$ have |
| 498 |
|
the lowest free energy of the studied polymorphs with all the models. |
| 499 |
|
Ideally, crystallization of each model under constant pressure |
| 500 |
|
conditions, as was done with SSD/E, would aid in the identification of |
| 503 |
|
insight about important behavior of others. |
| 504 |
|
|
| 505 |
|
We also note that none of the water models used in this study are |
| 506 |
< |
polarizable or flexible models. It is entirely possible that the |
| 507 |
< |
polarizability of real water makes Ice-{\it i} substantially less |
| 508 |
< |
stable than ice I$_h$. However, the calculations presented above seem |
| 509 |
< |
interesting enough to communicate before the role of polarizability |
| 510 |
< |
(or flexibility) has been thoroughly investigated. |
| 506 |
> |
polarizable or flexible models. It is entirely possible that the |
| 507 |
> |
polarizability of real water makes Ice-$i$ substantially less stable |
| 508 |
> |
than ice I$_\textrm{h}$. The dipole moment of the water molecules |
| 509 |
> |
increases as the system becomes more condensed, and the increasing |
| 510 |
> |
dipole moment should destabilize the tetramer structures in |
| 511 |
> |
Ice-$i$. Right now, using TIP4P-Ew with an electrostatic correction |
| 512 |
> |
gives the proper thermodynamically preferred state, and we recommend |
| 513 |
> |
this arrangement for study of crystallization processes if the |
| 514 |
> |
computational cost increase that comes with including polarizability |
| 515 |
> |
is an issue. |
| 516 |
|
|
| 517 |
< |
Finally, due to the stability of Ice-{\it i} in the investigated |
| 517 |
> |
Finally, due to the stability of Ice-$i$ in the investigated |
| 518 |
|
simulation conditions, the question arises as to possible experimental |
| 519 |
|
observation of this polymorph. The rather extensive past and current |
| 520 |
|
experimental investigation of water in the low pressure regime makes |
