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\begin{document} |
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\title{Ice-{\it i}: a simulation-predicted ice polymorph which is more |
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stable than Ice $I_h$ for point-charge and point-dipole water models} |
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|
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\author{Christopher J. Fennell and J. Daniel Gezelter \\ |
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Department of Chemistry and Biochemistry\\ University of Notre Dame\\ |
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Notre Dame, Indiana 46556} |
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\date{\today} |
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\maketitle |
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%\doublespacing |
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\begin{abstract} |
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The free energies of several ice polymorphs in the low pressure regime |
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were calculated using thermodynamic integration. These integrations |
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were done for most of the common water models. Ice-{\it i}, a |
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structure we recently observed to be stable in one of the single-point |
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water models, was determined to be the stable crystalline state (at 1 |
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atm) for {\it all} the water models investigated. Phase diagrams were |
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generated, and phase coexistence lines were determined for all of the |
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known low-pressure ice structures under all of the common water |
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models. Additionally, potential truncation was shown to have an |
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effect on the calculated free energies, and can result in altered free |
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energy landscapes. |
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\end{abstract} |
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%\narrowtext |
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% BODY OF TEXT |
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|
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\section{Introduction} |
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|
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Computer simulations are a valuable tool for studying the phase |
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behavior of systems ranging from small or simple molecules to complex |
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biological species.\cite{Matsumoto02,Li96,Marrink01} Useful techniques |
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have been developed to investigate the thermodynamic properites of |
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model substances, providing both qualitative and quantitative |
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comparisons between simulations and |
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experiment.\cite{Widom63,Frenkel84} Investigation of these properties |
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leads to the development of new and more accurate models, leading to |
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better understanding and depiction of physical processes and intricate |
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molecular systems. |
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|
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Water has proven to be a challenging substance to depict in |
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simulations, and a variety of models have been developed to describe |
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its behavior under varying simulation |
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conditions.\cite{Rahman75,Berendsen81,Jorgensen83,Bratko85,Berendsen87,Liu96,Berendsen98,Mahoney00,Fennell04} |
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These models have been used to investigate important physical |
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phenomena like phase transitions, molecule transport, and the |
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hydrophobic effect.\cite{Yamada02,Marrink94,Gallagher03} With the |
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choice of models available, it is only natural to compare the models |
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under interesting thermodynamic conditions in an attempt to clarify |
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the limitations of each of the |
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models.\cite{Jorgensen83,Jorgensen98b,Clancy94,Mahoney01} Two |
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important property to quantify are the Gibbs and Helmholtz free |
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energies, particularly for the solid forms of water. Difficulty in |
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these types of studies typically arises from the assortment of |
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possible crystalline polymorphs that water adopts over a wide range of |
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pressures and temperatures. There are currently 13 recognized forms |
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of ice, and it is a challenging task to investigate the entire free |
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energy landscape.\cite{Sanz04} Ideally, research is focused on the |
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phases having the lowest free energy at a given state point, because |
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these phases will dictate the true transition temperatures and |
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pressures for the model. |
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|
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In this paper, standard reference state methods were applied to known |
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crystalline water polymorphs in the low pressure regime. This work is |
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unique in the fact that one of the crystal lattices was arrived at |
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through crystallization of a computationally efficient water model |
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under constant pressure and temperature conditions. Crystallization |
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events are interesting in and of |
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themselves;\cite{Matsumoto02,Yamada02} however, the crystal structure |
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obtained in this case is different from any previously observed ice |
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polymorphs in experiment or simulation.\cite{Fennell04} We have named |
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this structure Ice-{\it i} to indicate its origin in computational |
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simulation. The unit cell (Fig. \ref{iceiCell}A) consists of eight |
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water molecules that stack in rows of interlocking water |
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tetramers. Proton ordering can be accomplished by orienting two of the |
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molecules so that both of their donated hydrogen bonds are internal to |
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their tetramer (Fig. \ref{protOrder}). As expected in an ice crystal |
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constructed of water tetramers, the hydrogen bonds are not as linear |
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as those observed in ice $I_h$, however the interlocking of these |
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subunits appears to provide significant stabilization to the overall |
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crystal. The arrangement of these tetramers results in surrounding |
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open octagonal cavities that are typically greater than 6.3 \AA\ in |
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diameter. This relatively open overall structure leads to crystals |
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that are 0.07 g/cm$^3$ less dense on average than ice $I_h$. |
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|
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\begin{figure} |
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\includegraphics[width=\linewidth]{unitCell.eps} |
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\caption{Unit cells for (A) Ice-{\it i} and (B) Ice-$i^\prime$, the |
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elongated variant of Ice-{\it i}. The spheres represent the |
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center-of-mass locations of the water molecules. The $a$ to $c$ |
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ratios for Ice-{\it i} and Ice-{\it i}$^\prime$ are given by |
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$a:2.1214c$ and $a:1.7850c$ respectively.} |
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\label{iceiCell} |
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\end{figure} |
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|
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\begin{figure} |
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\includegraphics[width=\linewidth]{orderedIcei.eps} |
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\caption{Image of a proton ordered crystal of Ice-{\it i} looking |
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down the (001) crystal face. The rows of water tetramers surrounded by |
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octagonal pores leads to a crystal structure that is significantly |
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less dense than ice $I_h$.} |
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\label{protOrder} |
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\end{figure} |
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|
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Results from our previous study indicated that Ice-{\it i} is the |
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minimum energy crystal structure for the single point water models we |
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investigated (for discussions on these single point dipole models, see |
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our previous work and related |
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articles).\cite{Fennell04,Liu96,Bratko85} Those results only |
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considered energetic stabilization and neglected entropic |
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contributions to the overall free energy. To address this issue, the |
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absolute free energy of this crystal was calculated using |
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thermodynamic integration and compared to the free energies of cubic |
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and hexagonal ice $I$ (the experimental low density ice polymorphs) |
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and ice B (a higher density, but very stable crystal structure |
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observed by B\`{a}ez and Clancy in free energy studies of |
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SPC/E).\cite{Baez95b} This work includes results for the water model |
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from which Ice-{\it i} was crystallized (SSD/E) in addition to several |
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common water models (TIP3P, TIP4P, TIP5P, and SPC/E) and a reaction |
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field parametrized single point dipole water model (SSD/RF). It should |
148 |
be noted that a second version of Ice-{\it i} (Ice-$i^\prime$) was used |
149 |
in calculations involving SPC/E, TIP4P, and TIP5P. The unit cell of |
150 |
this crystal (Fig. \ref{iceiCell}B) is similar to the Ice-{\it i} unit |
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it is extended in the direction of the (001) face and compressed along |
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the other two faces. |
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|
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\section{Methods} |
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|
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Canonical ensemble (NVT) molecular dynamics calculations were |
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performed using the OOPSE molecular mechanics package.\cite{Meineke05} |
158 |
All molecules were treated as rigid bodies, with orientational motion |
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propagated using the symplectic DLM integration method. Details about |
160 |
the implementation of this technique can be found in a recent |
161 |
publication.\cite{Dullweber1997} |
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|
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Thermodynamic integration is an established technique for |
164 |
determination of free energies of condensed phases of |
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materials.\cite{Frenkel84,Hermens88,Meijer90,Baez95a,Vlot99}. This |
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method, implemented in the same manner illustrated by B\`{a}ez and |
167 |
Clancy, was utilized to calculate the free energy of several ice |
168 |
crystals at 200 K using the TIP3P, TIP4P, TIP5P, SPC/E, SSD/RF, and |
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SSD/E water models.\cite{Baez95a} Liquid state free energies at 300 |
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and 400 K for all of these water models were also determined using |
171 |
this same technique in order to determine melting points and generate |
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phase diagrams. All simulations were carried out at densities |
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resulting in a pressure of approximately 1 atm at their respective |
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temperatures. |
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|
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A single thermodynamic integration involves a sequence of simulations |
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over which the system of interest is converted into a reference system |
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for which the free energy is known analytically. This transformation |
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path is then integrated in order to determine the free energy |
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difference between the two states: |
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\begin{equation} |
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\Delta A = \int_0^1\left\langle\frac{\partial V(\lambda |
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)}{\partial\lambda}\right\rangle_\lambda d\lambda, |
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\end{equation} |
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where $V$ is the interaction potential and $\lambda$ is the |
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transformation parameter that scales the overall |
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potential. Simulations are distributed strategically along this path |
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in order to sufficiently sample the regions of greatest change in the |
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potential. Typical integrations in this study consisted of $\sim$25 |
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simulations ranging from 300 ps (for the unaltered system) to 75 ps |
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(near the reference state) in length. |
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|
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For the thermodynamic integration of molecular crystals, the Einstein |
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crystal was chosen as the reference system. In an Einstein crystal, |
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the molecules are restrained at their ideal lattice locations and |
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orientations. Using harmonic restraints, as applied by B\`{a}ez and |
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Clancy, the total potential for this reference crystal |
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($V_\mathrm{EC}$) is the sum of all the harmonic restraints, |
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\begin{equation} |
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V_\mathrm{EC} = \frac{K_\mathrm{v}r^2}{2} + \frac{K_\theta\theta^2}{2} + |
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\frac{K_\omega\omega^2}{2}, |
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\end{equation} |
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where $K_\mathrm{v}$, $K_\mathrm{\theta}$, and $K_\mathrm{\omega}$ are |
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the spring constants restraining translational motion and deflection |
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of and rotation around the principle axis of the molecule |
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respectively. It is clear from Fig. \ref{waterSpring} that the values |
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of $\theta$ range from $0$ to $\pi$, while $\omega$ ranges from |
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$-\pi$ to $\pi$. The partition function for a molecular crystal |
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restrained in this fashion can be evaluated analytically, and the |
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Helmholtz Free Energy ({\it A}) is given by |
211 |
\begin{eqnarray} |
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A = E_m\ -\ kT\ln \left (\frac{kT}{h\nu}\right )^3&-&kT\ln \left |
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[\pi^\frac{1}{2}\left (\frac{8\pi^2I_\mathrm{A}kT}{h^2}\right |
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)^\frac{1}{2}\left (\frac{8\pi^2I_\mathrm{B}kT}{h^2}\right |
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)^\frac{1}{2}\left (\frac{8\pi^2I_\mathrm{C}kT}{h^2}\right |
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)^\frac{1}{2}\right ] \nonumber \\ &-& kT\ln \left [\frac{kT}{2(\pi |
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K_\omega K_\theta)^{\frac{1}{2}}}\exp\left |
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(-\frac{kT}{2K_\theta}\right)\int_0^{\left (\frac{kT}{2K_\theta}\right |
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)^\frac{1}{2}}\exp(t^2)\mathrm{d}t\right ], |
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\label{ecFreeEnergy} |
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\end{eqnarray} |
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where $2\pi\nu = (K_\mathrm{v}/m)^{1/2}$, and $E_m$ is the minimum |
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potential energy of the ideal crystal.\cite{Baez95a} |
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|
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\begin{figure} |
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\includegraphics[width=\linewidth]{rotSpring.eps} |
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\caption{Possible orientational motions for a restrained molecule. |
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$\theta$ angles correspond to displacement from the body-frame {\it |
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z}-axis, while $\omega$ angles correspond to rotation about the |
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body-frame {\it z}-axis. $K_\theta$ and $K_\omega$ are spring |
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constants for the harmonic springs restraining motion in the $\theta$ |
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and $\omega$ directions.} |
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\label{waterSpring} |
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\end{figure} |
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|
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In the case of molecular liquids, the ideal vapor is chosen as the |
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target reference state. There are several examples of liquid state |
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free energy calculations of water models present in the |
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literature.\cite{Hermens88,Quintana92,Mezei92,Baez95b} These methods |
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typically differ in regard to the path taken for switching off the |
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interaction potential to convert the system to an ideal gas of water |
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molecules. In this study, we apply of one of the most convenient |
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methods and integrate over the $\lambda^4$ path, where all interaction |
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parameters are scaled equally by this transformation parameter. This |
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method has been shown to be reversible and provide results in |
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excellent agreement with other established methods.\cite{Baez95b} |
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|
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Charge, dipole, and Lennard-Jones interactions were modified by a |
249 |
cubic switching between 100\% and 85\% of the cutoff value (9 \AA |
250 |
). By applying this function, these interactions are smoothly |
251 |
truncated, thereby avoiding the poor energy conservation which results |
252 |
from harsher truncation schemes. The effect of a long-range correction |
253 |
was also investigated on select model systems in a variety of |
254 |
manners. For the SSD/RF model, a reaction field with a fixed |
255 |
dielectric constant of 80 was applied in all |
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simulations.\cite{Onsager36} For a series of the least computationally |
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expensive models (SSD/E, SSD/RF, and TIP3P), simulations were |
258 |
performed with longer cutoffs of 12 and 15 \AA\ to compare with the 9 |
259 |
\AA\ cutoff results. Finally, results from the use of an Ewald |
260 |
summation were estimated for TIP3P and SPC/E by performing |
261 |
calculations with Particle-Mesh Ewald (PME) in the TINKER molecular |
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mechanics software package.\cite{Tinker} The calculated energy |
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difference in the presence and absence of PME was applied to the |
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previous results in order to predict changes to the free energy |
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landscape. |
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|
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\section{Results and discussion} |
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|
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The free energy of proton ordered Ice-{\it i} was calculated and |
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compared with the free energies of proton ordered variants of the |
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experimentally observed low-density ice polymorphs, $I_h$ and $I_c$, |
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as well as the higher density ice B, observed by B\`{a}ez and Clancy |
273 |
and thought to be the minimum free energy structure for the SPC/E |
274 |
model at ambient conditions (Table \ref{freeEnergy}).\cite{Baez95b} |
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Ice XI, the experimentally-observed proton-ordered variant of ice |
276 |
$I_h$, was investigated initially, but was found to be not as stable |
277 |
as proton disordered or antiferroelectric variants of ice $I_h$. The |
278 |
proton ordered variant of ice $I_h$ used here is a simple |
279 |
antiferroelectric version that we divised, and it has an 8 molecule |
280 |
unit cell similar to other predicted antiferroelectric $I_h$ |
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crystals.\cite{Davidson84} The crystals contained 648 or 1728 |
282 |
molecules for ice B, 1024 or 1280 molecules for ice $I_h$, 1000 |
283 |
molecules for ice $I_c$, or 1024 molecules for Ice-{\it i}. The larger |
284 |
crystal sizes were necessary for simulations involving larger cutoff |
285 |
values. |
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|
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\begin{table*} |
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\begin{minipage}{\linewidth} |
289 |
\renewcommand{\thefootnote}{\thempfootnote} |
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\begin{center} |
291 |
\caption{Calculated free energies for several ice polymorphs with a |
292 |
variety of common water models. All calculations used a cutoff radius |
293 |
of 9 \AA\ and were performed at 200 K and $\sim$1 atm. Units are |
294 |
kcal/mol. Calculated error of the final digits is in parentheses. *Ice |
295 |
$I_c$ rapidly converts to a liquid at 200 K with the SSD/RF model.} |
296 |
\begin{tabular}{ l c c c c } |
297 |
\hline |
298 |
Water Model & $I_h$ & $I_c$ & B & Ice-{\it i}\\ |
299 |
\hline |
300 |
TIP3P & -11.41(2) & -11.23(3) & -11.82(3) & -12.30(3)\\ |
301 |
TIP4P & -11.84(3) & -12.04(2) & -12.08(3) & -12.33(3)\\ |
302 |
TIP5P & -11.85(3) & -11.86(2) & -11.96(2) & -12.29(2)\\ |
303 |
SPC/E & -12.67(2) & -12.96(2) & -13.25(3) & -13.55(2)\\ |
304 |
SSD/E & -11.27(2) & -11.19(4) & -12.09(2) & -12.54(2)\\ |
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SSD/RF & -11.51(2) & NA* & -12.08(3) & -12.29(2)\\ |
306 |
\end{tabular} |
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\label{freeEnergy} |
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\end{center} |
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\end{minipage} |
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\end{table*} |
311 |
|
312 |
The free energy values computed for the studied polymorphs indicate |
313 |
that Ice-{\it i} is the most stable state for all of the common water |
314 |
models studied. With the free energy at these state points, the |
315 |
Gibbs-Helmholtz equation was used to project to other state points and |
316 |
to build phase diagrams. Figures |
317 |
\ref{tp3phasedia} and \ref{ssdrfphasedia} are example diagrams built |
318 |
from the free energy results. All other models have similar structure, |
319 |
although the crossing points between the phases exist at slightly |
320 |
different temperatures and pressures. It is interesting to note that |
321 |
ice $I$ does not exist in either cubic or hexagonal form in any of the |
322 |
phase diagrams for any of the models. For purposes of this study, ice |
323 |
B is representative of the dense ice polymorphs. A recent study by |
324 |
Sanz {\it et al.} goes into detail on the phase diagrams for SPC/E and |
325 |
TIP4P in the high pressure regime.\cite{Sanz04} |
326 |
|
327 |
\begin{figure} |
328 |
\includegraphics[width=\linewidth]{tp3PhaseDia.eps} |
329 |
\caption{Phase diagram for the TIP3P water model in the low pressure |
330 |
regime. The displayed $T_m$ and $T_b$ values are good predictions of |
331 |
the experimental values; however, the solid phases shown are not the |
332 |
experimentally observed forms. Both cubic and hexagonal ice $I$ are |
333 |
higher in energy and don't appear in the phase diagram.} |
334 |
\label{tp3phasedia} |
335 |
\end{figure} |
336 |
|
337 |
\begin{figure} |
338 |
\includegraphics[width=\linewidth]{ssdrfPhaseDia.eps} |
339 |
\caption{Phase diagram for the SSD/RF water model in the low pressure |
340 |
regime. Calculations producing these results were done under an |
341 |
applied reaction field. It is interesting to note that this |
342 |
computationally efficient model (over 3 times more efficient than |
343 |
TIP3P) exhibits phase behavior similar to the less computationally |
344 |
conservative charge based models.} |
345 |
\label{ssdrfphasedia} |
346 |
\end{figure} |
347 |
|
348 |
\begin{table*} |
349 |
\begin{minipage}{\linewidth} |
350 |
\renewcommand{\thefootnote}{\thempfootnote} |
351 |
\begin{center} |
352 |
\caption{Melting ($T_m$), boiling ($T_b$), and sublimation ($T_s$) |
353 |
temperatures at 1 atm for several common water models compared with |
354 |
experiment. The $T_m$ and $T_s$ values from simulation correspond to a |
355 |
transition between Ice-{\it i} (or Ice-{\it i}$^\prime$) and the |
356 |
liquid or gas state.} |
357 |
\begin{tabular}{ l c c c c c c c } |
358 |
\hline |
359 |
Equilibria Point & TIP3P & TIP4P & TIP5P & SPC/E & SSD/E & SSD/RF & Exp.\\ |
360 |
\hline |
361 |
$T_m$ (K) & 269(4) & 266(5) & 271(4) & 296(3) & - & 278(4) & 273\\ |
362 |
$T_b$ (K) & 357(2) & 354(2) & 337(2) & 396(2) & - & 348(2) & 373\\ |
363 |
$T_s$ (K) & - & - & - & - & 355(2) & - & -\\ |
364 |
\end{tabular} |
365 |
\label{meltandboil} |
366 |
\end{center} |
367 |
\end{minipage} |
368 |
\end{table*} |
369 |
|
370 |
Table \ref{meltandboil} lists the melting and boiling temperatures |
371 |
calculated from this work. Surprisingly, most of these models have |
372 |
melting points that compare quite favorably with experiment. The |
373 |
unfortunate aspect of this result is that this phase change occurs |
374 |
between Ice-{\it i} and the liquid state rather than ice $I_h$ and the |
375 |
liquid state. These results are actually not contrary to previous |
376 |
studies in the literature. Earlier free energy studies of ice $I$ |
377 |
using TIP4P predict a $T_m$ ranging from 214 to 238 K (differences |
378 |
being attributed to choice of interaction truncation and different |
379 |
ordered and disordered molecular |
380 |
arrangements).\cite{Vlot99,Gao00,Sanz04} If the presence of ice B and |
381 |
Ice-{\it i} were omitted, a $T_m$ value around 210 K would be |
382 |
predicted from this work. However, the $T_m$ from Ice-{\it i} is |
383 |
calculated at 265 K, significantly higher in temperature than the |
384 |
previous studies. Also of interest in these results is that SSD/E does |
385 |
not exhibit a melting point at 1 atm, but it shows a sublimation point |
386 |
at 355 K. This is due to the significant stability of Ice-{\it i} over |
387 |
all other polymorphs for this particular model under these |
388 |
conditions. While troubling, this behavior turned out to be |
389 |
advantageous in that it facilitated the spontaneous crystallization of |
390 |
Ice-{\it i}. These observations provide a warning that simulations of |
391 |
SSD/E as a ``liquid'' near 300 K are actually metastable and run the |
392 |
risk of spontaneous crystallization. However, this risk changes when |
393 |
applying a longer cutoff. |
394 |
|
395 |
\begin{figure} |
396 |
\includegraphics[width=\linewidth]{cutoffChange.eps} |
397 |
\caption{Free energy as a function of cutoff radius for (A) SSD/E, (B) |
398 |
TIP3P, and (C) SSD/RF. Data points omitted include SSD/E: $I_c$ 12 |
399 |
\AA\, TIP3P: $I_c$ 12 \AA\ and B 12 \AA\, and SSD/RF: $I_c$ 9 |
400 |
\AA . These crystals are unstable at 200 K and rapidly convert into |
401 |
liquids. The connecting lines are qualitative visual aid.} |
402 |
\label{incCutoff} |
403 |
\end{figure} |
404 |
|
405 |
Increasing the cutoff radius in simulations of the more |
406 |
computationally efficient water models was done in order to evaluate |
407 |
the trend in free energy values when moving to systems that do not |
408 |
involve potential truncation. As seen in Fig. \ref{incCutoff}, the |
409 |
free energy of all the ice polymorphs show a substantial dependence on |
410 |
cutoff radius. In general, there is a narrowing of the free energy |
411 |
differences while moving to greater cutoff radius. Interestingly, by |
412 |
increasing the cutoff radius, the free energy gap was narrowed enough |
413 |
in the SSD/E model that the liquid state is preferred under standard |
414 |
simulation conditions (298 K and 1 atm). Thus, it is recommended that |
415 |
simulations using this model choose interaction truncation radii |
416 |
greater than 9 \AA\ . This narrowing trend is much more subtle in the |
417 |
case of SSD/RF, indicating that the free energies calculated with a |
418 |
reaction field present provide a more accurate picture of the free |
419 |
energy landscape in the absence of potential truncation. |
420 |
|
421 |
To further study the changes resulting to the inclusion of a |
422 |
long-range interaction correction, the effect of an Ewald summation |
423 |
was estimated by applying the potential energy difference do to its |
424 |
inclusion in systems in the presence and absence of the |
425 |
correction. This was accomplished by calculation of the potential |
426 |
energy of identical crystals with and without PME using TINKER. The |
427 |
free energies for the investigated polymorphs using the TIP3P and |
428 |
SPC/E water models are shown in Table \ref{pmeShift}. The same trend |
429 |
pointed out through increase of cutoff radius is observed in these PME |
430 |
results. Ice-{\it i} is the preferred polymorph at ambient conditions |
431 |
for both the TIP3P and SPC/E water models; however, the narrowing of |
432 |
the free energy differences between the various solid forms is |
433 |
significant enough that it becomes less clear that it is the most |
434 |
stable polymorph with the SPC/E model. The free energies of Ice-{\it |
435 |
i} and ice B nearly overlap within error, with ice $I_c$ just outside |
436 |
as well, indicating that Ice-{\it i} might be metastable with respect |
437 |
to ice B and possibly ice $I_c$ with SPC/E. However, these results do |
438 |
not significantly alter the finding that the Ice-{\it i} polymorph is |
439 |
a stable crystal structure that should be considered when studying the |
440 |
phase behavior of water models. |
441 |
|
442 |
\begin{table*} |
443 |
\begin{minipage}{\linewidth} |
444 |
\renewcommand{\thefootnote}{\thempfootnote} |
445 |
\begin{center} |
446 |
\caption{The free energy of the studied ice polymorphs after applying |
447 |
the energy difference attributed to the inclusion of the PME |
448 |
long-range interaction correction. Units are kcal/mol.} |
449 |
\begin{tabular}{ l c c c c } |
450 |
\hline |
451 |
\ \ Water Model \ \ & \ \ \ \ \ $I_h$ \ \ & \ \ \ \ \ $I_c$ \ \ & \ \quad \ \ \ \ B \ \ & \ \ \ \ \ Ice-{\it i} \ \ \\ |
452 |
\hline |
453 |
TIP3P & -11.53(2) & -11.24(3) & -11.51(3) & -11.67(3)\\ |
454 |
SPC/E & -12.77(2) & -12.92(2) & -12.96(3) & -13.02(2)\\ |
455 |
\end{tabular} |
456 |
\label{pmeShift} |
457 |
\end{center} |
458 |
\end{minipage} |
459 |
\end{table*} |
460 |
|
461 |
\section{Conclusions} |
462 |
|
463 |
The free energy for proton ordered variants of hexagonal and cubic ice |
464 |
$I$, ice B, and recently discovered Ice-{\it i} were calculated under |
465 |
standard conditions for several common water models via thermodynamic |
466 |
integration. All the water models studied show Ice-{\it i} to be the |
467 |
minimum free energy crystal structure in the with a 9 \AA\ switching |
468 |
function cutoff. Calculated melting and boiling points show |
469 |
surprisingly good agreement with the experimental values; however, the |
470 |
solid phase at 1 atm is Ice-{\it i}, not ice $I_h$. The effect of |
471 |
interaction truncation was investigated through variation of the |
472 |
cutoff radius, use of a reaction field parameterized model, and |
473 |
estimation of the results in the presence of the Ewald |
474 |
summation. Interaction truncation has a significant effect on the |
475 |
computed free energy values, and may significantly alter the free |
476 |
energy landscape for the more complex multipoint water models. Despite |
477 |
these effects, these results show Ice-{\it i} to be an important ice |
478 |
polymorph that should be considered in simulation studies. |
479 |
|
480 |
Due to this relative stability of Ice-{\it i} in all manner of |
481 |
investigated simulation examples, the question arises as to possible |
482 |
experimental observation of this polymorph. The rather extensive past |
483 |
and current experimental investigation of water in the low pressure |
484 |
regime makes us hesitant to ascribe any relevance of this work outside |
485 |
of the simulation community. It is for this reason that we chose a |
486 |
name for this polymorph which involves an imaginary quantity. That |
487 |
said, there are certain experimental conditions that would provide the |
488 |
most ideal situation for possible observation. These include the |
489 |
negative pressure or stretched solid regime, small clusters in vacuum |
490 |
deposition environments, and in clathrate structures involving small |
491 |
non-polar molecules. Figs. \ref{fig:gofr} and \ref{fig:sofq} contain |
492 |
our predictions for both the pair distribution function ($g_{OO}(r)$) |
493 |
and the structure factor ($S(\vec{q})$ for ice $I_c$ and for ice-{\it |
494 |
i} at a temperature of 77K. In a quick comparison of the predicted |
495 |
S(q) for Ice-{\it i} and experimental studies of amorphous solid |
496 |
water, it is possible that some of the ``spurious'' peaks that could |
497 |
not be assigned in HDA 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 |
500 |
such peak comparisons should be made with caution. We will leave it |
501 |
to our experimental colleagues to determine whether this ice polymorph |
502 |
is named appropriately or if it should be promoted to Ice-0. |
503 |
|
504 |
\begin{figure} |
505 |
\includegraphics[width=\linewidth]{iceGofr.eps} |
506 |
\caption{Radial distribution functions of Ice-{\it i} and ice $I_c$ |
507 |
calculated from from simulations of the SSD/RF water model at 77 K.} |
508 |
\label{fig:gofr} |
509 |
\end{figure} |
510 |
|
511 |
\begin{figure} |
512 |
\includegraphics[width=\linewidth]{sofq.eps} |
513 |
\caption{Predicted structure factors for Ice-{\it i} and ice $I_c$ at |
514 |
77 K. The raw structure factors have been convoluted with a gaussian |
515 |
instrument function (0.075 \AA$^{-1}$ width) to compensate for the |
516 |
trunction effects in our finite size simulations. The labeled peaks |
517 |
compared favorably with ``spurious'' peaks observed in experimental |
518 |
studies of amorphous solid water.\cite{Bizid87}} |
519 |
\label{fig:sofq} |
520 |
\end{figure} |
521 |
|
522 |
\section{Acknowledgments} |
523 |
Support for this project was provided by the National Science |
524 |
Foundation under grant CHE-0134881. Computation time was provided by |
525 |
the Notre Dame High Performance Computing Cluster and the Notre Dame |
526 |
Bunch-of-Boxes (B.o.B) computer cluster (NSF grant DMR-0079647). |
527 |
|
528 |
\newpage |
529 |
|
530 |
\bibliographystyle{jcp} |
531 |
\bibliography{iceiPaper} |
532 |
|
533 |
|
534 |
\end{document} |