| 56 |
|
|
| 57 |
|
Molecular dynamics is a valuable tool for studying the phase behavior |
| 58 |
|
of systems ranging from small or simple |
| 59 |
< |
molecules\cite{Matsumoto02andOthers} to complex biological |
| 59 |
> |
molecules\cite{Matsumoto02,andOthers} to complex biological |
| 60 |
|
species.\cite{bigStuff} Many techniques have been developed to |
| 61 |
|
investigate the thermodynamic properites of model substances, |
| 62 |
|
providing both qualitative and quantitative comparisons between |
| 130 |
|
minimum energy crystal structure for the single point water models we |
| 131 |
|
investigated (for discussions on these single point dipole models, see |
| 132 |
|
the previous work and related |
| 133 |
< |
articles\cite{Fennell04,Ichiye96,Bratko85}). Those results only |
| 133 |
> |
articles\cite{Fennell04,Liu96,Bratko85}). Those results only |
| 134 |
|
considered energetic stabilization and neglected entropic |
| 135 |
|
contributions to the overall free energy. To address this issue, the |
| 136 |
|
absolute free energy of this crystal was calculated using |
| 155 |
|
All molecules were treated as rigid bodies, with orientational motion |
| 156 |
|
propagated using the symplectic DLM integration method. Details about |
| 157 |
|
the implementation of these techniques can be found in a recent |
| 158 |
< |
publication.\cite{DLM} |
| 158 |
> |
publication.\cite{Dullweber1997} |
| 159 |
|
|
| 160 |
|
Thermodynamic integration was utilized to calculate the free energy of |
| 161 |
|
several ice crystals at 200 K using the TIP3P, TIP4P, TIP5P, SPC/E, |
| 385 |
|
in the SSD/E model that the liquid state is preferred under standard |
| 386 |
|
simulation conditions (298 K and 1 atm). Thus, it is recommended that |
| 387 |
|
simulations using this model choose interaction truncation radii |
| 388 |
< |
greater than 9 \AA\. This narrowing trend is much more subtle in the |
| 388 |
> |
greater than 9 \AA\ . This narrowing trend is much more subtle in the |
| 389 |
|
case of SSD/RF, indicating that the free energies calculated with a |
| 390 |
|
reaction field present provide a more accurate picture of the free |
| 391 |
|
energy landscape in the absence of potential truncation. |
| 460 |
|
most ideal situation for possible observation. These include the |
| 461 |
|
negative pressure or stretched solid regime, small clusters in vacuum |
| 462 |
|
deposition environments, and in clathrate structures involving small |
| 463 |
< |
non-polar molecules. Fig. \ref{fig:gofr} contains our predictions |
| 464 |
< |
of both the pair distribution function ($g_{OO}(r)$) and the structure |
| 465 |
< |
factor ($S(\vec{q})$ for this polymorph at a temperature of 77K. We |
| 466 |
< |
will leave it to our experimental colleagues to determine whether this |
| 467 |
< |
ice polymorph should really be called Ice-{\it i} or if it should be |
| 468 |
< |
promoted to Ice-0. |
| 463 |
> |
non-polar molecules. Figs. \ref{fig:gofr} and \ref{fig:sofq} contain |
| 464 |
> |
our predictions for both the pair distribution function ($g_{OO}(r)$) |
| 465 |
> |
and the structure factor ($S(\vec{q})$ for ice $I_c$ and for ice-{\it |
| 466 |
> |
i} at a temperature of 77K. In a quick comparison of the predicted |
| 467 |
> |
S(q) for Ice-{\it i} and experimental studies of amorphous solid |
| 468 |
> |
water, it is possible that some of the ``spurious'' peaks that could |
| 469 |
> |
not be assigned in HDA could correspond to peaks labeled in this |
| 470 |
> |
S(q).\cite{Bizid87} It should be noted that there is typically poor |
| 471 |
> |
agreement on crystal densities between simulation and experiment, so |
| 472 |
> |
such peak comparisons should be made with caution. We will leave it |
| 473 |
> |
to our experimental colleagues to determine whether this ice polymorph |
| 474 |
> |
is named appropriately or if it should be promoted to Ice-0. |
| 475 |
|
|
| 476 |
|
\begin{figure} |
| 477 |
|
\includegraphics[width=\linewidth]{iceGofr.eps} |
| 478 |
< |
\caption{Radial distribution functions of (A) Ice-{\it i} and (B) ice $I_c$ at 77 K from simulations of the SSD/RF water model.} |
| 478 |
> |
\caption{Radial distribution functions of Ice-{\it i} and ice $I_c$ |
| 479 |
> |
calculated from from simulations of the SSD/RF water model at 77 K.} |
| 480 |
|
\label{fig:gofr} |
| 481 |
|
\end{figure} |
| 482 |
|
|
| 483 |
+ |
\begin{figure} |
| 484 |
+ |
\includegraphics[width=\linewidth]{sofq.eps} |
| 485 |
+ |
\caption{Predicted structure factors for Ice-{\it i} and ice $I_c$ at |
| 486 |
+ |
77 K. The raw structure factors have been convoluted with a gaussian |
| 487 |
+ |
instrument function (0.075 \AA$^{-1}$ width) to compensate for the |
| 488 |
+ |
trunction effects in our finite size simulations. The labeled peaks |
| 489 |
+ |
compared favorably with ``spurious'' peaks observed in experimental |
| 490 |
+ |
studies of amorphous solid water.\cite{Bizid87}} |
| 491 |
+ |
\label{fig:sofq} |
| 492 |
+ |
\end{figure} |
| 493 |
+ |
|
| 494 |
|
\section{Acknowledgments} |
| 495 |
|
Support for this project was provided by the National Science |
| 496 |
|
Foundation under grant CHE-0134881. Computation time was provided by |
