| 61 |
|
Water has proven to be a challenging substance to depict in |
| 62 |
|
simulations, and a variety of models have been developed to describe |
| 63 |
|
its behavior under varying simulation |
| 64 |
< |
conditions.\cite{Rahman75,Berendsen81,Jorgensen83,Bratko85,Berendsen87,Liu96,Berendsen98,Mahoney00,Fennell04,Amoeba,POL3} |
| 64 |
> |
conditions.\cite{Stillinger74,Rahman75,Berendsen81,Jorgensen83,Bratko85,Berendsen87,Caldwell95,Liu96,Berendsen98,Dill00,Mahoney00,Fennell04} |
| 65 |
|
These models have been used to investigate important physical |
| 66 |
|
phenomena like phase transitions, transport properties, and the |
| 67 |
|
hydrophobic effect.\cite{Yamada02,Marrink94,Gallagher03} With the |
| 105 |
|
|
| 106 |
|
\begin{figure} |
| 107 |
|
\includegraphics[width=\linewidth]{unitCell.eps} |
| 108 |
< |
\caption{Unit cells for (A) Ice-{\it i} and (B) Ice-$i^\prime$, the |
| 109 |
< |
elongated variant of Ice-{\it i}. The spheres represent the |
| 108 |
> |
\caption{Unit cells for (A) Ice-{\it i} and (B) Ice-{\it i}$^\prime$, |
| 109 |
> |
the elongated variant of Ice-{\it i}. The spheres represent the |
| 110 |
|
center-of-mass locations of the water molecules. The $a$ to $c$ |
| 111 |
|
ratios for Ice-{\it i} and Ice-{\it i}$^\prime$ are given by |
| 112 |
|
$a:2.1214c$ and $a:1.7850c$ respectively.} |
| 138 |
|
from which Ice-{\it i} was crystallized (SSD/E) in addition to several |
| 139 |
|
common water models (TIP3P, TIP4P, TIP5P, and SPC/E) and a reaction |
| 140 |
|
field parametrized single point dipole water model (SSD/RF). It should |
| 141 |
< |
be noted that a second version of Ice-{\it i} (Ice-$i^\prime$) was |
| 142 |
< |
used in calculations involving SPC/E, TIP4P, and TIP5P. The unit cell |
| 143 |
< |
of this crystal (Fig. \ref{iceiCell}B) is similar to the Ice-{\it i} |
| 144 |
< |
unit it is extended in the direction of the (001) face and compressed |
| 145 |
< |
along the other two faces. |
| 141 |
> |
be noted that a second version of Ice-{\it i} (Ice-{\it i}$^\prime$) |
| 142 |
> |
was used in calculations involving SPC/E, TIP4P, and TIP5P. The unit |
| 143 |
> |
cell of this crystal (Fig. \ref{iceiCell}B) is similar to the Ice-{\it |
| 144 |
> |
i} unit it is extended in the direction of the (001) face and |
| 145 |
> |
compressed along the other two faces. There is typically a small unit |
| 146 |
> |
cell distortion of Ice-{\it i}$^\prime$ that converts the normally |
| 147 |
> |
square tetramer into a rhombus with alternating 85 and 95 degree |
| 148 |
> |
angles. The degree of this distortion is model dependent and |
| 149 |
> |
significant enough to split the tetramer diagonal location peak in the |
| 150 |
> |
radial distibution function. |
| 151 |
|
|
| 152 |
|
\section{Methods} |
| 153 |
|
|
| 489 |
|
deposition environments, and in clathrate structures involving small |
| 490 |
|
non-polar molecules. Figs. \ref{fig:gofr} and \ref{fig:sofq} contain |
| 491 |
|
our predictions for both the pair distribution function ($g_{OO}(r)$) |
| 492 |
< |
and the structure factor ($S(\vec{q})$ for ice $I_c$ and for ice-{\it |
| 493 |
< |
i} at a temperature of 77K. In our initial comparison of the |
| 494 |
< |
predicted S(q) for Ice-{\it i} and experimental studies of amorphous |
| 495 |
< |
solid water, it is possible that some of the ``spurious'' peaks that |
| 496 |
< |
could not be assigned in an early report on high-density amorphous |
| 497 |
< |
(HDA) ice could correspond to peaks labeled in this |
| 498 |
< |
S(q).\cite{Bizid87} It should be noted that there is typically poor |
| 499 |
< |
agreement on crystal densities between simulation and experiment, so |
| 495 |
< |
such peak comparisons should be made with caution. We will leave it |
| 496 |
< |
to our experimental colleagues to make the final determination on |
| 497 |
< |
whether this ice polymorph is named appropriately (i.e. with an |
| 498 |
< |
imaginary number) or if it can be promoted to Ice-0. |
| 492 |
> |
and the structure factor ($S(\vec{q})$ for ice $I_h$, $I_c$, and for |
| 493 |
> |
ice-{\it i} at a temperature of 77K. In studies of the high and low |
| 494 |
> |
density forms of amorphous ice, ``spurious'' diffraction peaks have |
| 495 |
> |
been observed experimentally.\cite{Bizid87} It is possible that a |
| 496 |
> |
variant of Ice-{\it i} could explain some of this behavior; however, |
| 497 |
> |
we will leave it to our experimental colleagues to make the final |
| 498 |
> |
determination on whether this ice polymorph is named appropriately |
| 499 |
> |
(i.e. with an imaginary number) or if it can be promoted to Ice-0. |
| 500 |
|
|
| 501 |
|
\begin{figure} |
| 502 |
|
\includegraphics[width=\linewidth]{iceGofr.eps} |
| 503 |
< |
\caption{Radial distribution functions of Ice-{\it i} and ice $I_c$ |
| 504 |
< |
calculated from from simulations of the SSD/RF water model at 77 K.} |
| 503 |
> |
\caption{Radial distribution functions of ice $I_h$, $I_c$, |
| 504 |
> |
Ice-{\it i}, and Ice-{\it i}$^\prime$ calculated from from simulations |
| 505 |
> |
of the SSD/RF water model at 77 K.} |
| 506 |
|
\label{fig:gofr} |
| 507 |
|
\end{figure} |
| 508 |
|
|
| 509 |
|
\begin{figure} |
| 510 |
|
\includegraphics[width=\linewidth]{sofq.eps} |
| 511 |
< |
\caption{Predicted structure factors for Ice-{\it i} and ice $I_c$ at |
| 512 |
< |
77 K. The raw structure factors have been convoluted with a gaussian |
| 513 |
< |
instrument function (0.075 \AA$^{-1}$ width) to compensate for the |
| 514 |
< |
trunction effects in our finite size simulations. The labeled peaks |
| 515 |
< |
compared favorably with ``spurious'' peaks observed in experimental |
| 514 |
< |
studies of amorphous solid water.\cite{Bizid87}} |
| 511 |
> |
\caption{Predicted structure factors for ice $I_h$, $I_c$, Ice-{\it i}, |
| 512 |
> |
and Ice-{\it i}$^\prime$ at 77 K. The raw structure factors have |
| 513 |
> |
been convoluted with a gaussian instrument function (0.075 \AA$^{-1}$ |
| 514 |
> |
width) to compensate for the trunction effects in our finite size |
| 515 |
> |
simulations.} |
| 516 |
|
\label{fig:sofq} |
| 517 |
|
\end{figure} |
| 518 |
|
|
