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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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\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 absolute free energies of several ice polymorphs which are stable |
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at low pressures were calculated using thermodynamic integration to a |
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reference system (the Einstein crystal). These integrations were |
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performed for most of the common water models (SPC/E, TIP3P, TIP4P, |
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TIP5P, SSD/E, SSD/RF). Ice-{\it i}, a structure we recently observed |
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crystallizing at room temperature for one of the single-point water |
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models, was determined to be the stable crystalline state (at 1 atm) |
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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 these water models. |
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Additionally, potential truncation was shown to have an effect on the |
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calculated free energies, and can result in altered free energy |
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landscapes. Structure factor predictions for the new crystal were |
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generated and we await experimental confirmation of the existence of |
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this new polymorph. |
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\end{abstract} |
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%\narrowtext |
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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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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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{Stillinger74,Rahman75,Berendsen81,Jorgensen83,Bratko85,Berendsen87,Caldwell95,Liu96,Berendsen98,Dill00,Mahoney00,Fennell04} |
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These models have been used to investigate important physical |
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phenomena like phase transitions, transport properties, 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 properties 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 relevant 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 that one of the crystal lattices was arrived at through |
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crystallization of a computationally efficient water model under |
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constant pressure and temperature conditions. Crystallization events |
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are interesting in and of themselves;\cite{Matsumoto02,Yamada02} |
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however, the crystal structure obtained in this case is different from |
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any previously observed ice polymorphs in experiment or |
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simulation.\cite{Fennell04} We have named this structure Ice-{\it i} |
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to indicate its origin in computational simulation. The unit cell |
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(Fig. \ref{iceiCell}A) consists of eight water molecules that stack in |
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rows of interlocking water tetramers. Proton ordering can be |
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accomplished by orienting two of the molecules so that both of their |
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donated hydrogen bonds are internal to their tetramer |
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(Fig. \ref{protOrder}). As expected in an ice crystal constructed of |
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water tetramers, the hydrogen bonds are not as linear as those |
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observed in ice $I_h$, however the interlocking of these subunits |
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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-{\it i}$^\prime$, |
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the 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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had investigated (for discussions on these single point dipole models, |
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see 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, we |
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have calculated the absolute free energy of this crystal 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 |
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be noted that a second version of Ice-{\it i} (Ice-{\it i}$^\prime$) |
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was used in calculations involving SPC/E, TIP4P, and TIP5P. The unit |
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cell of this crystal (Fig. \ref{iceiCell}B) is similar to the Ice-{\it |
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i} unit it is extended in the direction of the (001) face and |
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compressed along the other two faces. There is typically a small |
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distortion of proton ordered Ice-{\it i}$^\prime$ that converts the |
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normally square tetramer into a rhombus with alternating approximately |
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85 and 95 degree angles. The degree of this distortion is model |
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dependent and significant enough to split the tetramer diagonal |
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location peak in the radial distribution function. |
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|
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\section{Methods} |
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Canonical ensemble (NVT) molecular dynamics calculations were |
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performed using the OOPSE molecular mechanics package.\cite{Meineke05} |
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All molecules were treated as rigid bodies, with orientational motion |
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propagated using the symplectic DLM integration method. Details about |
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the implementation of this technique can be found in a recent |
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publication.\cite{Dullweber1997} |
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|
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Thermodynamic integration is an established technique for |
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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 |
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Clancy, was utilized to calculate the free energy of several ice |
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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 |
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this same technique in order to determine melting points and to |
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generate phase diagrams. All simulations were carried out at densities |
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which correspond to a pressure of approximately 1 atm at their |
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respective temperatures. |
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|
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Thermodynamic integration involves a sequence of simulations during |
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which the system of interest is converted into a reference system for |
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which the free energy is known analytically. This transformation path |
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is then integrated in order to determine the free energy difference |
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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. These spring constants are typically calculated from |
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the mean-square displacements of water molecules in an unrestrained |
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ice crystal at 200 K. For these studies, $K_\mathrm{r} = 4.29$ kcal |
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mol$^{-1}$, $K_\theta\ = 13.88$ kcal mol$^{-1}$, and $K_\omega\ = |
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17.75$ kcal mol$^{-1}$. It is clear from Fig. \ref{waterSpring} that |
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the values of $\theta$ range from $0$ to $\pi$, while $\omega$ ranges |
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from $-\pi$ 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 |
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\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 applied of one of the most convenient |
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methods and integrated over the $\lambda^4$ path, where all |
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interaction parameters are scaled equally by this transformation |
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parameter. This method has been shown to be reversible and provide |
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results in excellent agreement with other established |
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methods.\cite{Baez95b} |
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|
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Charge, dipole, and Lennard-Jones interactions were modified by a |
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cubic switching between 100\% and 85\% of the cutoff value (9 \AA |
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). By applying this function, these interactions are smoothly |
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truncated, thereby avoiding the poor energy conservation which results |
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from harsher truncation schemes. The effect of a long-range correction |
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was also investigated on select model systems in a variety of |
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manners. For the SSD/RF model, a reaction field with a fixed |
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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 |
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performed with longer cutoffs of 12 and 15 \AA\ to compare with the 9 |
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\AA\ cutoff results. Finally, the effects of utilizing an Ewald |
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summation were estimated for TIP3P and SPC/E by performing single |
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configuration calculations with Particle-Mesh Ewald (PME) in the |
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TINKER molecular mechanics software package.\cite{Tinker} The |
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calculated energy difference in the presence and absence of PME was |
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applied to the previous results in order to predict changes to the |
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free energy 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 |
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and thought to be the minimum free energy structure for the SPC/E |
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model at ambient conditions (Table \ref{freeEnergy}).\cite{Baez95b} |
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Ice XI, the experimentally-observed proton-ordered variant of ice |
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$I_h$, was investigated initially, but was found to be not as stable |
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as proton disordered or antiferroelectric variants of ice $I_h$. The |
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proton ordered variant of ice $I_h$ used here is a simple |
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antiferroelectric version that we devised, and it has an 8 molecule |
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unit cell similar to other predicted antiferroelectric $I_h$ |
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crystals.\cite{Davidson84} The crystals contained 648 or 1728 |
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molecules for ice B, 1024 or 1280 molecules for ice $I_h$, 1000 |
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molecules for ice $I_c$, or 1024 molecules for Ice-{\it i}. The larger |
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crystal sizes were necessary for simulations involving larger cutoff |
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values. |
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|
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\begin{table*} |
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\begin{minipage}{\linewidth} |
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\begin{center} |
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|
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\caption{Calculated free energies for several ice polymorphs with a |
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variety of common water models. All calculations used a cutoff radius |
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of 9 \AA\ and were performed at 200 K and $\sim$1 atm. Units are |
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kcal/mol. Calculated error of the final digits is in parentheses.} |
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|
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\begin{tabular}{lcccc} |
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\hline |
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Water Model & $I_h$ & $I_c$ & B & Ice-{\it i}\\ |
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\hline |
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TIP3P & -11.41(2) & -11.23(3) & -11.82(3) & -12.30(3)\\ |
304 |
|
|
TIP4P & -11.84(3) & -12.04(2) & -12.08(3) & -12.33(3)\\ |
305 |
|
|
TIP5P & -11.85(3) & -11.86(2) & -11.96(2) & -12.29(2)\\ |
306 |
chrisfen |
1542 |
SPC/E & -12.87(2) & -13.05(2) & -13.26(3) & -13.55(2)\\ |
307 |
chrisfen |
1473 |
SSD/E & -11.27(2) & -11.19(4) & -12.09(2) & -12.54(2)\\ |
308 |
chrisfen |
1528 |
SSD/RF & -11.51(2) & -11.47(2) & -12.08(3) & -12.29(2)\\ |
309 |
chrisfen |
1456 |
\end{tabular} |
310 |
|
|
\label{freeEnergy} |
311 |
|
|
\end{center} |
312 |
|
|
\end{minipage} |
313 |
|
|
\end{table*} |
314 |
chrisfen |
1453 |
|
315 |
chrisfen |
1456 |
The free energy values computed for the studied polymorphs indicate |
316 |
|
|
that Ice-{\it i} is the most stable state for all of the common water |
317 |
gezelter |
1475 |
models studied. With the calculated free energy at these state points, |
318 |
|
|
the Gibbs-Helmholtz equation was used to project to other state points |
319 |
|
|
and to build phase diagrams. Figures |
320 |
chrisfen |
1456 |
\ref{tp3phasedia} and \ref{ssdrfphasedia} are example diagrams built |
321 |
|
|
from the free energy results. All other models have similar structure, |
322 |
gezelter |
1475 |
although the crossing points between the phases move to slightly |
323 |
gezelter |
1465 |
different temperatures and pressures. It is interesting to note that |
324 |
|
|
ice $I$ does not exist in either cubic or hexagonal form in any of the |
325 |
|
|
phase diagrams for any of the models. For purposes of this study, ice |
326 |
|
|
B is representative of the dense ice polymorphs. A recent study by |
327 |
|
|
Sanz {\it et al.} goes into detail on the phase diagrams for SPC/E and |
328 |
gezelter |
1475 |
TIP4P at higher pressures than those studied here.\cite{Sanz04} |
329 |
gezelter |
1463 |
|
330 |
chrisfen |
1456 |
\begin{figure} |
331 |
|
|
\includegraphics[width=\linewidth]{tp3PhaseDia.eps} |
332 |
|
|
\caption{Phase diagram for the TIP3P water model in the low pressure |
333 |
|
|
regime. The displayed $T_m$ and $T_b$ values are good predictions of |
334 |
|
|
the experimental values; however, the solid phases shown are not the |
335 |
|
|
experimentally observed forms. Both cubic and hexagonal ice $I$ are |
336 |
|
|
higher in energy and don't appear in the phase diagram.} |
337 |
|
|
\label{tp3phasedia} |
338 |
|
|
\end{figure} |
339 |
gezelter |
1463 |
|
340 |
chrisfen |
1456 |
\begin{figure} |
341 |
|
|
\includegraphics[width=\linewidth]{ssdrfPhaseDia.eps} |
342 |
|
|
\caption{Phase diagram for the SSD/RF water model in the low pressure |
343 |
|
|
regime. Calculations producing these results were done under an |
344 |
|
|
applied reaction field. It is interesting to note that this |
345 |
|
|
computationally efficient model (over 3 times more efficient than |
346 |
|
|
TIP3P) exhibits phase behavior similar to the less computationally |
347 |
|
|
conservative charge based models.} |
348 |
|
|
\label{ssdrfphasedia} |
349 |
|
|
\end{figure} |
350 |
|
|
|
351 |
|
|
\begin{table*} |
352 |
|
|
\begin{minipage}{\linewidth} |
353 |
|
|
\begin{center} |
354 |
gezelter |
1489 |
|
355 |
chrisfen |
1456 |
\caption{Melting ($T_m$), boiling ($T_b$), and sublimation ($T_s$) |
356 |
chrisfen |
1466 |
temperatures at 1 atm for several common water models compared with |
357 |
|
|
experiment. The $T_m$ and $T_s$ values from simulation correspond to a |
358 |
|
|
transition between Ice-{\it i} (or Ice-{\it i}$^\prime$) and the |
359 |
|
|
liquid or gas state.} |
360 |
gezelter |
1489 |
|
361 |
|
|
\begin{tabular}{lccccccc} |
362 |
gezelter |
1463 |
\hline |
363 |
gezelter |
1489 |
Equilibrium Point & TIP3P & TIP4P & TIP5P & SPC/E & SSD/E & SSD/RF & Exp.\\ |
364 |
gezelter |
1463 |
\hline |
365 |
chrisfen |
1473 |
$T_m$ (K) & 269(4) & 266(5) & 271(4) & 296(3) & - & 278(4) & 273\\ |
366 |
|
|
$T_b$ (K) & 357(2) & 354(2) & 337(2) & 396(2) & - & 348(2) & 373\\ |
367 |
|
|
$T_s$ (K) & - & - & - & - & 355(2) & - & -\\ |
368 |
chrisfen |
1456 |
\end{tabular} |
369 |
|
|
\label{meltandboil} |
370 |
|
|
\end{center} |
371 |
|
|
\end{minipage} |
372 |
|
|
\end{table*} |
373 |
|
|
|
374 |
|
|
Table \ref{meltandboil} lists the melting and boiling temperatures |
375 |
|
|
calculated from this work. Surprisingly, most of these models have |
376 |
|
|
melting points that compare quite favorably with experiment. The |
377 |
|
|
unfortunate aspect of this result is that this phase change occurs |
378 |
|
|
between Ice-{\it i} and the liquid state rather than ice $I_h$ and the |
379 |
chrisfen |
1806 |
liquid state. These results are actually not contrary to other |
380 |
|
|
studies. Studies of ice $I_h$ using TIP4P predict a $T_m$ ranging from |
381 |
|
|
214 to 238 K (differences being attributed to choice of interaction |
382 |
|
|
truncation and different ordered and disordered molecular |
383 |
chrisfen |
1466 |
arrangements).\cite{Vlot99,Gao00,Sanz04} If the presence of ice B and |
384 |
|
|
Ice-{\it i} were omitted, a $T_m$ value around 210 K would be |
385 |
chrisfen |
1456 |
predicted from this work. However, the $T_m$ from Ice-{\it i} is |
386 |
chrisfen |
1806 |
calculated to be 265 K, indicating that these simulation based |
387 |
|
|
structures ought to be included in studies probing phase transitions |
388 |
|
|
with this model. Also of interest in these results is that SSD/E does |
389 |
chrisfen |
1456 |
not exhibit a melting point at 1 atm, but it shows a sublimation point |
390 |
|
|
at 355 K. This is due to the significant stability of Ice-{\it i} over |
391 |
|
|
all other polymorphs for this particular model under these |
392 |
gezelter |
1475 |
conditions. While troubling, this behavior resulted in spontaneous |
393 |
|
|
crystallization of Ice-{\it i} and led us to investigate this |
394 |
|
|
structure. These observations provide a warning that simulations of |
395 |
chrisfen |
1456 |
SSD/E as a ``liquid'' near 300 K are actually metastable and run the |
396 |
gezelter |
1475 |
risk of spontaneous crystallization. However, this risk lessens when |
397 |
chrisfen |
1456 |
applying a longer cutoff. |
398 |
|
|
|
399 |
chrisfen |
1458 |
\begin{figure} |
400 |
|
|
\includegraphics[width=\linewidth]{cutoffChange.eps} |
401 |
chrisfen |
1806 |
\caption{Free energy as a function of cutoff radius for SSD/E, TIP3P, |
402 |
|
|
SPC/E, SSD/RF with a reaction field, and the TIP3P and SPC/E models |
403 |
|
|
with an added Ewald correction term. Calculations performed without a |
404 |
|
|
long-range correction show noticable free energy dependence on the |
405 |
|
|
cutoff radius and show some degree of converge at large cutoff |
406 |
|
|
radii. Inclusion of a long-range correction reduces the cutoff radius |
407 |
|
|
dependence of the free energy for all the models. Data for ice I$_c$ |
408 |
|
|
with TIP3P using 12 and 13.5 \AA\ cutoff radii were omitted being that |
409 |
|
|
the crystal was prone to distortion and melting at 200 K.} |
410 |
chrisfen |
1458 |
\label{incCutoff} |
411 |
|
|
\end{figure} |
412 |
|
|
|
413 |
chrisfen |
1457 |
Increasing the cutoff radius in simulations of the more |
414 |
|
|
computationally efficient water models was done in order to evaluate |
415 |
|
|
the trend in free energy values when moving to systems that do not |
416 |
|
|
involve potential truncation. As seen in Fig. \ref{incCutoff}, the |
417 |
chrisfen |
1806 |
free energy of the ice polymorphs with water models lacking a |
418 |
|
|
long-range correction show a cutoff radius dependence. In general, |
419 |
|
|
there is a narrowing of the free energy differences while moving to |
420 |
|
|
greater cutoff radii. As the free energies for the polymorphs |
421 |
|
|
converge, the stability advantage that Ice-{\it i} exhibits is |
422 |
|
|
reduced; however, it remains the most stable polymorph for both of |
423 |
|
|
these models over the depicted range for both models. This narrowing |
424 |
|
|
trend is not significant in the case of SSD/RF, indicating that the |
425 |
|
|
free energies calculated with a reaction field present provide, at |
426 |
|
|
minimal computational cost, a more accurate picture of the free energy |
427 |
chrisfen |
1528 |
landscape in the absence of potential truncation. Interestingly, |
428 |
chrisfen |
1806 |
increasing the cutoff radius a mere 1.5 |
429 |
|
|
\AA\ with the SSD/E model destabilizes the Ice-{\it i} polymorph |
430 |
|
|
enough that the liquid state is preferred under standard simulation |
431 |
|
|
conditions (298 K and 1 atm). Thus, it is recommended that simulations |
432 |
|
|
using this model choose interaction truncation radii greater than 9 |
433 |
|
|
\AA. Considering this stabilization provided by smaller cutoffs, it is |
434 |
|
|
not surprising that crystallization into Ice-{\it i} was observed with |
435 |
|
|
SSD/E. The choice of a 9 \AA\ cutoff in the previous simulations |
436 |
|
|
gives the Ice-{\it i} polymorph a greater than 1 kcal/mol lower free |
437 |
|
|
energy than the ice $I_\textrm{h}$ starting configurations. |
438 |
chrisfen |
1456 |
|
439 |
chrisfen |
1457 |
To further study the changes resulting to the inclusion of a |
440 |
|
|
long-range interaction correction, the effect of an Ewald summation |
441 |
|
|
was estimated by applying the potential energy difference do to its |
442 |
chrisfen |
1542 |
inclusion in systems in the presence and absence of the correction. |
443 |
|
|
This was accomplished by calculation of the potential energy of |
444 |
|
|
identical crystals both with and without PME. The free energies for |
445 |
|
|
the investigated polymorphs using the TIP3P and SPC/E water models are |
446 |
|
|
shown in Table \ref{pmeShift}. The same trend pointed out through |
447 |
|
|
increase of cutoff radius is observed in these PME results. Ice-{\it |
448 |
|
|
i} is the preferred polymorph at ambient conditions for both the TIP3P |
449 |
|
|
and SPC/E water models; however, the narrowing of the free energy |
450 |
|
|
differences between the various solid forms with the SPC/E model is |
451 |
chrisfen |
1471 |
significant enough that it becomes less clear that it is the most |
452 |
chrisfen |
1542 |
stable polymorph. The free energies of Ice-{\it i} and $I_\textrm{c}$ |
453 |
|
|
overlap within error, while ice B and $I_\textrm{h}$ are just outside |
454 |
|
|
at t slightly higher free energy. This indicates that with SPC/E, |
455 |
|
|
Ice-{\it i} might be metastable with all the studied polymorphs, |
456 |
|
|
particularly ice $I_\textrm{c}$. However, these results do not |
457 |
|
|
significantly alter the finding that the Ice-{\it i} polymorph is a |
458 |
|
|
stable crystal structure that should be considered when studying the |
459 |
chrisfen |
1474 |
phase behavior of water models. |
460 |
chrisfen |
1456 |
|
461 |
chrisfen |
1457 |
\begin{table*} |
462 |
|
|
\begin{minipage}{\linewidth} |
463 |
|
|
\begin{center} |
464 |
gezelter |
1489 |
|
465 |
chrisfen |
1458 |
\caption{The free energy of the studied ice polymorphs after applying |
466 |
|
|
the energy difference attributed to the inclusion of the PME |
467 |
|
|
long-range interaction correction. Units are kcal/mol.} |
468 |
gezelter |
1489 |
|
469 |
|
|
\begin{tabular}{ccccc} |
470 |
gezelter |
1463 |
\hline |
471 |
gezelter |
1489 |
Water Model & $I_h$ & $I_c$ & B & Ice-{\it i} \\ |
472 |
gezelter |
1463 |
\hline |
473 |
gezelter |
1489 |
TIP3P & -11.53(2) & -11.24(3) & -11.51(3) & -11.67(3) \\ |
474 |
chrisfen |
1542 |
SPC/E & -12.97(2) & -13.00(2) & -12.96(3) & -13.02(2) \\ |
475 |
chrisfen |
1457 |
\end{tabular} |
476 |
|
|
\label{pmeShift} |
477 |
|
|
\end{center} |
478 |
|
|
\end{minipage} |
479 |
|
|
\end{table*} |
480 |
|
|
|
481 |
chrisfen |
1453 |
\section{Conclusions} |
482 |
|
|
|
483 |
chrisfen |
1458 |
The free energy for proton ordered variants of hexagonal and cubic ice |
484 |
gezelter |
1475 |
$I$, ice B, and our recently discovered Ice-{\it i} structure were |
485 |
|
|
calculated under standard conditions for several common water models |
486 |
|
|
via thermodynamic integration. All the water models studied show |
487 |
|
|
Ice-{\it i} to be the minimum free energy crystal structure with a 9 |
488 |
|
|
\AA\ switching function cutoff. Calculated melting and boiling points |
489 |
|
|
show surprisingly good agreement with the experimental values; |
490 |
|
|
however, the solid phase at 1 atm is Ice-{\it i}, not ice $I_h$. The |
491 |
|
|
effect of interaction truncation was investigated through variation of |
492 |
|
|
the cutoff radius, use of a reaction field parameterized model, and |
493 |
gezelter |
1465 |
estimation of the results in the presence of the Ewald |
494 |
|
|
summation. Interaction truncation has a significant effect on the |
495 |
chrisfen |
1459 |
computed free energy values, and may significantly alter the free |
496 |
|
|
energy landscape for the more complex multipoint water models. Despite |
497 |
|
|
these effects, these results show Ice-{\it i} to be an important ice |
498 |
|
|
polymorph that should be considered in simulation studies. |
499 |
chrisfen |
1458 |
|
500 |
gezelter |
1475 |
Due to this relative stability of Ice-{\it i} in all of the |
501 |
|
|
investigated simulation conditions, the question arises as to possible |
502 |
gezelter |
1465 |
experimental observation of this polymorph. The rather extensive past |
503 |
chrisfen |
1459 |
and current experimental investigation of water in the low pressure |
504 |
gezelter |
1465 |
regime makes us hesitant to ascribe any relevance of this work outside |
505 |
|
|
of the simulation community. It is for this reason that we chose a |
506 |
|
|
name for this polymorph which involves an imaginary quantity. That |
507 |
|
|
said, there are certain experimental conditions that would provide the |
508 |
|
|
most ideal situation for possible observation. These include the |
509 |
|
|
negative pressure or stretched solid regime, small clusters in vacuum |
510 |
|
|
deposition environments, and in clathrate structures involving small |
511 |
gezelter |
1469 |
non-polar molecules. Figs. \ref{fig:gofr} and \ref{fig:sofq} contain |
512 |
|
|
our predictions for both the pair distribution function ($g_{OO}(r)$) |
513 |
chrisfen |
1479 |
and the structure factor ($S(\vec{q})$ for ice $I_h$, $I_c$, and for |
514 |
|
|
ice-{\it i} at a temperature of 77K. In studies of the high and low |
515 |
|
|
density forms of amorphous ice, ``spurious'' diffraction peaks have |
516 |
|
|
been observed experimentally.\cite{Bizid87} It is possible that a |
517 |
|
|
variant of Ice-{\it i} could explain some of this behavior; however, |
518 |
|
|
we will leave it to our experimental colleagues to make the final |
519 |
|
|
determination on whether this ice polymorph is named appropriately |
520 |
|
|
(i.e. with an imaginary number) or if it can be promoted to Ice-0. |
521 |
chrisfen |
1459 |
|
522 |
chrisfen |
1467 |
\begin{figure} |
523 |
|
|
\includegraphics[width=\linewidth]{iceGofr.eps} |
524 |
chrisfen |
1479 |
\caption{Radial distribution functions of ice $I_h$, $I_c$, |
525 |
|
|
Ice-{\it i}, and Ice-{\it i}$^\prime$ calculated from from simulations |
526 |
|
|
of the SSD/RF water model at 77 K.} |
527 |
chrisfen |
1467 |
\label{fig:gofr} |
528 |
|
|
\end{figure} |
529 |
|
|
|
530 |
gezelter |
1469 |
\begin{figure} |
531 |
|
|
\includegraphics[width=\linewidth]{sofq.eps} |
532 |
chrisfen |
1479 |
\caption{Predicted structure factors for ice $I_h$, $I_c$, Ice-{\it i}, |
533 |
|
|
and Ice-{\it i}$^\prime$ at 77 K. The raw structure factors have |
534 |
|
|
been convoluted with a gaussian instrument function (0.075 \AA$^{-1}$ |
535 |
|
|
width) to compensate for the trunction effects in our finite size |
536 |
|
|
simulations.} |
537 |
gezelter |
1469 |
\label{fig:sofq} |
538 |
|
|
\end{figure} |
539 |
|
|
|
540 |
chrisfen |
1453 |
\section{Acknowledgments} |
541 |
|
|
Support for this project was provided by the National Science |
542 |
|
|
Foundation under grant CHE-0134881. Computation time was provided by |
543 |
chrisfen |
1458 |
the Notre Dame High Performance Computing Cluster and the Notre Dame |
544 |
|
|
Bunch-of-Boxes (B.o.B) computer cluster (NSF grant DMR-0079647). |
545 |
chrisfen |
1453 |
|
546 |
|
|
\newpage |
547 |
|
|
|
548 |
|
|
\bibliographystyle{jcp} |
549 |
|
|
\bibliography{iceiPaper} |
550 |
|
|
|
551 |
|
|
|
552 |
|
|
\end{document} |