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\begin{document} |
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\title{A Free Energy Study of Low Temperature and Anomalous Ice} |
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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{\thefootnote} |
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\footnote[1]{Corresponding author. \ Electronic mail: gezelter@nd.edu}} |
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\address{Department of Chemistry and Biochemistry\\ University of Notre Dame\\ |
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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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\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 of systems consisting |
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of a variety of common water models. Ice-{\it i}, a recent |
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computationally observed solid structure, was determined to be the |
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stable state with the lowest free energy for all the water models |
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investigated. Phase diagrams were generated, and melting and boiling |
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points for all the models were determined and show relatively good |
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agreement with experiment, although the solid phase is different |
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between simulation and experiment. In addition, potential truncation |
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was shown to have an effect on the calculated free energies, and may |
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result in altered free energy landscapes. |
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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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\maketitle |
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\newpage |
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%\narrowtext |
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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\section{Introduction} |
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|
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Molecular dynamics has developed into a valuable tool for studying the |
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phase behavior of systems ranging from small or simple |
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molecules\cite{smallStuff} to complex biological |
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species.\cite{bigStuff} Many techniques have been developed in order |
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to investigate the thermodynamic properites of model substances, |
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Molecular dynamics is a valuable tool for studying the phase behavior |
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of systems ranging from small or simple |
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molecules\cite{Matsumoto02andOthers} to complex biological |
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species.\cite{bigStuff} Many techniques have been developed to |
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investigate the thermodynamic properites of model substances, |
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providing both qualitative and quantitative comparisons between |
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simulations and experiment.\cite{thermMethods} Investigation of these |
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properties leads to the development of new and more accurate models, |
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and intricate molecular systems. |
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Water has proven to be a challenging substance to depict in |
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simulations, and has resulted in a variety of models that attempt to |
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describe its behavior under a varying simulation |
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conditions.\cite{lotsOfWaterPapers} Many of these models have been |
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used to investigate important physical phenomena like phase |
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transitions and the hydrophobic effect.\cite{evenMorePapers} With the |
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advent of numerous differing models, it is only natural that attention |
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is placed on the properties of the models themselves in an attempt to |
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clarify their benefits and limitations when applied to a system of |
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interest.\cite{modelProps} One important but challenging property to |
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quantify is the free energy, particularly of the solid forms of |
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water. Difficulty in these types of studies typically arises from the |
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assortment of possible crystalline polymorphs that water that water |
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adopts over a wide range of pressures and temperatures. There are |
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currently 13 recognized forms of ice, and it is a challenging task to |
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investigate the entire free energy landscape.\cite{Sanz04} Ideally, |
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research is focused on the phases having the lowest free energy, |
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because these phases will dictate the true transition temperatures and |
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pressures for their respective model. |
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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{Berendsen81,Jorgensen83,Bratko85,Berendsen87,Liu96,Mahoney00,Fennell04} |
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These models have been used to investigate important physical |
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phenomena like phase transitions and the hydrophobic |
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effect.\cite{Yamada02} With the choice of models available, it |
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is only natural to compare the models under interesting thermodynamic |
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conditions in an attempt to clarify the limitations of each of the |
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models.\cite{modelProps} Two important property to quantify are the |
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Gibbs and Helmholtz free energies, particularly for the solid forms of |
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water. Difficulty in these types of studies typically arises from the |
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assortment of possible crystalline polymorphs that water adopts over a |
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wide range of pressures and temperatures. There are currently 13 |
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recognized forms of ice, and it is a challenging task to investigate |
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the entire free energy landscape.\cite{Sanz04} Ideally, research is |
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focused on the phases having the lowest free energy at a given state |
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point, because these phases will dictate the true transition |
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temperatures and pressures for their respective model. |
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|
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In this paper, standard reference state methods were applied to the |
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study of crystalline water polymorphs in the low pressure regime. This |
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work is unique in the fact that one of the crystal lattices was |
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arrived at through crystallization of a computationally efficient |
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water model under constant pressure and temperature |
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conditions. Crystallization events are interesting in and of |
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themselves\cite{nucleationStudies}; however, the crystal structure |
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obtained in this case was different from any previously observed ice |
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polymorphs, in experiment or simulation.\cite{Fennell04} This crystal |
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was termed Ice-{\it i} in homage to its origin in computational |
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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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waters so that both of their donated hydrogen bonds are internal to |
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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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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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\begin{figure} |
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\includegraphics[scale=1.0]{unitCell.eps} |
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\caption{Unit cells for (A) Ice-{\it i} and (B) Ice-2{\it i}, the elongated variant of Ice-{\it i}. For Ice-{\it i}, the $a$ to $c$ relation is given by $a = 1.0607c$, while for Ice-2{\it i}, $a = 0.8925c$.} |
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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}. For Ice-{\it i}, the $a$ to $c$ |
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relation is given by $a = 2.1214c$, while for Ice-$i^\prime$, $a = |
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1.7850c$.} |
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\label{iceiCell} |
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\end{figure} |
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\begin{figure} |
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\includegraphics[scale=1.0]{orderedIcei.eps} |
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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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\label{protOrder} |
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\end{figure} |
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Results in the previous study indicated that Ice-{\it i} is the |
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minimum energy crystal structure for the single point water models |
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being studied (for discussions on these single point dipole models, |
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see the previous work and related |
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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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the previous work and related |
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articles\cite{Fennell04,Ichiye96,Bratko85}). Those results only |
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consider energetic stabilization and neglect entropic contributions to |
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the overall free energy. To address this issue, the absolute free |
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energy of this crystal was calculated using thermodynamic integration |
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and compared to the free energies of cubic and hexagonal ice $I$ (the |
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experimental low density ice polymorphs) and ice B (a higher density, |
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but very stable crystal structure observed by B\`{a}ez and Clancy in |
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free energy studies of SPC/E).\cite{Baez95b} This work includes |
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results for the water model from which Ice-{\it i} was crystallized |
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(soft sticky dipole extended, SSD/E) in addition to several common |
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water models (TIP3P, TIP4P, TIP5P, and SPC/E) and a reaction field |
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parametrized single point dipole water model (soft sticky dipole |
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reaction field, SSD/RF). In should be noted that a second version of |
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Ice-{\it i} (Ice-2{\it i}) was used in calculations involving SPC/E, |
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TIP4P, and TIP5P. The unit cell of this crystal (Fig. \ref{iceiCell}B) |
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is similar to the Ice-{\it i} unit it is extended in the direction of |
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the (001) face and compressed along the other two faces. |
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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 |
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be noted that a second version of Ice-{\it i} (Ice-$i^\prime$) was used |
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in calculations involving SPC/E, TIP4P, and TIP5P. The unit cell of |
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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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\section{Methods} |
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Canonical ensemble (NVT) molecular dynamics calculations were |
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performed using the OOPSE (Object-Oriented Parallel Simulation Engine) |
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molecular mechanics package. All molecules were treated as rigid |
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bodies, with orientational motion propagated using the symplectic DLM |
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integration method. Details about the implementation of these |
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techniques can be found in a recent publication.\cite{Meineke05} |
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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 these techniques can be found in a recent |
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publication.\cite{DLM} |
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Thermodynamic integration was utilized to calculate the free energy of |
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several ice crystals at 200 K using the TIP3P, TIP4P, TIP5P, SPC/E, |
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SSD/RF, and SSD/E water models. Liquid state free energies at 300 and |
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400 K for all of these water models were also determined using this |
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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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same technique in order to determine melting points and generate phase |
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diagrams. All simulations were carried out at densities resulting in a |
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pressure of approximately 1 atm at their respective temperatures. |
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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. This transformation path is then |
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integrated in order to determine the free energy difference between |
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the two states: |
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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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(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 is chosen as the reference state that the system is converted |
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to over the course of the simulation. In an Einstein Crystal, the |
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crystal was chosen as the reference state. In an Einstein crystal, the |
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molecules are harmonically restrained at their ideal lattice locations |
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and orientations. The partition function for a molecular crystal |
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restrained in this fashion has been evaluated, and the Helmholtz Free |
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Energy ({\it A}) is given by |
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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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minimum potential energy of the ideal crystal. In the case of |
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molecular liquids, the ideal vapor is chosen as the target reference |
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state. |
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\begin{figure} |
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\includegraphics[scale=1.0]{rotSpring.eps} |
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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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\end{figure} |
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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 ). By |
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applying this function, these interactions are smoothly truncated, |
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thereby avoiding poor energy conserving dynamics resulting from |
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harsher truncation schemes. The effect of a long-range correction was |
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also investigated on select model systems in a variety of manners. For |
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the SSD/RF model, a reaction field with a fixed dielectric constant of |
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80 was applied in all simulations.\cite{Onsager36} For a series of the |
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least computationally expensive models (SSD/E, SSD/RF, and TIP3P), |
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simulations were performed with longer cutoffs of 12 and 15 \AA\ to |
| 232 |
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compare with the 9 \AA\ cutoff results. Finally, results from the use |
| 233 |
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of an Ewald summation were estimated for TIP3P and SPC/E by performing |
| 223 |
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cubic switching between 100\% and 85\% of the cutoff value (9 \AA |
| 224 |
> |
). By applying this function, these interactions are smoothly |
| 225 |
> |
truncated, thereby avoiding the poor energy conservation which results |
| 226 |
> |
from harsher truncation schemes. The effect of a long-range correction |
| 227 |
> |
was also investigated on select model systems in a variety of |
| 228 |
> |
manners. For the SSD/RF model, a reaction field with a fixed |
| 229 |
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dielectric constant of 80 was applied in all |
| 230 |
> |
simulations.\cite{Onsager36} For a series of the least computationally |
| 231 |
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expensive models (SSD/E, SSD/RF, and TIP3P), simulations were |
| 232 |
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performed with longer cutoffs of 12 and 15 \AA\ to compare with the 9 |
| 233 |
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\AA\ cutoff results. Finally, results from the use of an Ewald |
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summation were estimated for TIP3P and SPC/E by performing |
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calculations with Particle-Mesh Ewald (PME) in the TINKER molecular |
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mechanics software package. TINKER was chosen because it can also |
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propagate the motion of rigid-bodies, and provides the most direct |
| 238 |
< |
comparison to the results from OOPSE. The calculated energy difference |
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in the presence and absence of PME was applied to the previous results |
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in order to predict changes in the free energy landscape. |
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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 |
| 238 |
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previous results in order to predict changes to the free energy |
| 239 |
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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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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} |
| 249 |
< |
Ice XI, the experimentally observed proton ordered variant of ice |
| 250 |
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$I_h$, was investigated initially, but it was found not to be as |
| 251 |
< |
stable as antiferroelectric variants of proton ordered or even proton |
| 252 |
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disordered ice$I_h$.\cite{Davidson84} The proton ordered variant of |
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ice $I_h$ used here is a simple antiferroelectric version that has an |
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8 molecule unit cell. The crystals contained 648 or 1728 molecules for |
| 255 |
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ice B, 1024 or 1280 molecules for ice $I_h$, 1000 molecules for ice |
| 256 |
< |
$I_c$, or 1024 molecules for Ice-{\it i}. The larger crystal sizes |
| 249 |
> |
Ice XI, the experimentally-observed proton-ordered variant of ice |
| 250 |
> |
$I_h$, was investigated initially, but was found to be not as stable |
| 251 |
> |
as proton disordered or antiferroelectric variants of ice $I_h$. The |
| 252 |
> |
proton ordered variant of ice $I_h$ used here is a simple |
| 253 |
> |
antiferroelectric version that has an 8 molecule unit |
| 254 |
> |
cell.\cite{Davidson84} The crystals contained 648 or 1728 molecules |
| 255 |
> |
for ice B, 1024 or 1280 molecules for ice $I_h$, 1000 molecules for |
| 256 |
> |
ice $I_c$, or 1024 molecules for Ice-{\it i}. The larger crystal sizes |
| 257 |
|
were necessary for simulations involving larger cutoff values. |
| 258 |
|
|
| 259 |
|
\begin{table*} |
| 265 |
|
of 9 \AA\ and were performed at 200 K and $\sim$1 atm. Units are |
| 266 |
|
kcal/mol. *Ice $I_c$ is unstable at 200 K using SSD/RF.} |
| 267 |
|
\begin{tabular}{ l c c c c } |
| 268 |
< |
\hline \\[-7mm] |
| 268 |
> |
\hline |
| 269 |
|
\ \quad \ Water Model\ \ & \ \quad \ \ \ \ $I_h$ \ \ & \ \quad \ \ \ \ $I_c$ \ \ & \ \quad \ \ \ \ B \ \ & \ \quad \ \ \ Ice-{\it i} \ \quad \ \\ |
| 270 |
< |
\hline \\[-3mm] |
| 270 |
> |
\hline |
| 271 |
|
\ \quad \ TIP3P & \ \quad \ -11.41 & \ \quad \ -11.23 & \ \quad \ -11.82 & \quad -12.30\\ |
| 272 |
|
\ \quad \ TIP4P & \ \quad \ -11.84 & \ \quad \ -12.04 & \ \quad \ -12.08 & \quad -12.33\\ |
| 273 |
|
\ \quad \ TIP5P & \ \quad \ -11.85 & \ \quad \ -11.86 & \ \quad \ -11.96 & \quad -12.29\\ |
| 283 |
|
The free energy values computed for the studied polymorphs indicate |
| 284 |
|
that Ice-{\it i} is the most stable state for all of the common water |
| 285 |
|
models studied. With the free energy at these state points, the |
| 286 |
< |
temperature and pressure dependence of the free energy was used to |
| 287 |
< |
project to other state points and build phase diagrams. Figures |
| 286 |
> |
Gibbs-Helmholtz equation was used to project to other state points and |
| 287 |
> |
to build phase diagrams. Figures |
| 288 |
|
\ref{tp3phasedia} and \ref{ssdrfphasedia} are example diagrams built |
| 289 |
|
from the free energy results. All other models have similar structure, |
| 290 |
< |
only the crossing points between these phases exist at different |
| 291 |
< |
temperatures and pressures. It is interesting to note that ice $I$ |
| 292 |
< |
does not exist in either cubic or hexagonal form in any of the phase |
| 293 |
< |
diagrams for any of the models. For purposes of this study, ice B is |
| 294 |
< |
representative of the dense ice polymorphs. A recent study by Sanz |
| 295 |
< |
{\it et al.} goes into detail on the phase diagrams for SPC/E and |
| 296 |
< |
TIP4P in the high pressure regime.\cite{Sanz04} |
| 290 |
> |
although the crossing points between the phases exist at slightly |
| 291 |
> |
different temperatures and pressures. It is interesting to note that |
| 292 |
> |
ice $I$ does not exist in either cubic or hexagonal form in any of the |
| 293 |
> |
phase diagrams for any of the models. For purposes of this study, ice |
| 294 |
> |
B is representative of the dense ice polymorphs. A recent study by |
| 295 |
> |
Sanz {\it et al.} goes into detail on the phase diagrams for SPC/E and |
| 296 |
> |
TIP4P in the high pressure regime.\cite{Sanz04} |
| 297 |
> |
|
| 298 |
|
\begin{figure} |
| 299 |
|
\includegraphics[width=\linewidth]{tp3PhaseDia.eps} |
| 300 |
|
\caption{Phase diagram for the TIP3P water model in the low pressure |
| 304 |
|
higher in energy and don't appear in the phase diagram.} |
| 305 |
|
\label{tp3phasedia} |
| 306 |
|
\end{figure} |
| 307 |
+ |
|
| 308 |
|
\begin{figure} |
| 309 |
|
\includegraphics[width=\linewidth]{ssdrfPhaseDia.eps} |
| 310 |
|
\caption{Phase diagram for the SSD/RF water model in the low pressure |
| 323 |
|
\caption{Melting ($T_m$), boiling ($T_b$), and sublimation ($T_s$) |
| 324 |
|
temperatures of several common water models compared with experiment.} |
| 325 |
|
\begin{tabular}{ l c c c c c c c } |
| 326 |
< |
\hline \\[-7mm] |
| 326 |
> |
\hline |
| 327 |
|
\ \ Equilibria Point\ \ & \ \ \ \ \ TIP3P \ \ & \ \ \ \ \ TIP4P \ \ & \ \quad \ \ \ \ TIP5P \ \ & \ \ \ \ \ SPC/E \ \ & \ \ \ \ \ SSD/E \ \ & \ \ \ \ \ SSD/RF \ \ & \ \ \ \ \ Exp. \ \ \\ |
| 328 |
< |
\hline \\[-3mm] |
| 328 |
> |
\hline |
| 329 |
|
\ \ $T_m$ (K) & \ \ 269 & \ \ 265 & \ \ 271 & 297 & \ \ - & \ \ 278 & \ \ 273\\ |
| 330 |
|
\ \ $T_b$ (K) & \ \ 357 & \ \ 354 & \ \ 337 & 396 & \ \ - & \ \ 349 & \ \ 373\\ |
| 331 |
|
\ \ $T_s$ (K) & \ \ - & \ \ - & \ \ - & - & \ \ 355 & \ \ - & \ \ -\\ |
| 399 |
|
preferred polymorph at ambient conditions for both the TIP3P and SPC/E |
| 400 |
|
water models; however, there is a narrowing of the free energy |
| 401 |
|
differences between the various solid forms. In the case of SPC/E this |
| 402 |
< |
narrowing is significant enough that it becomes less clear cut that |
| 402 |
> |
narrowing is significant enough that it becomes less clear that |
| 403 |
|
Ice-{\it i} is the most stable polymorph, and is possibly metastable |
| 404 |
|
with respect to ice B and possibly ice $I_c$. However, these results |
| 405 |
|
do not significantly alter the finding that the Ice-{\it i} polymorph |
| 414 |
|
the energy difference attributed to the inclusion of the PME |
| 415 |
|
long-range interaction correction. Units are kcal/mol.} |
| 416 |
|
\begin{tabular}{ l c c c c } |
| 417 |
< |
\hline \\[-7mm] |
| 417 |
> |
\hline |
| 418 |
|
\ \ Water Model \ \ & \ \ \ \ \ $I_h$ \ \ & \ \ \ \ \ $I_c$ \ \ & \ \quad \ \ \ \ B \ \ & \ \ \ \ \ Ice-{\it i} \ \ \\ |
| 419 |
< |
\hline \\[-3mm] |
| 419 |
> |
\hline |
| 420 |
|
\ \ TIP3P & \ \ -11.53 & \ \ -11.24 & \ \ -11.51 & \ \ -11.67\\ |
| 421 |
|
\ \ SPC/E & \ \ -12.77 & \ \ -12.92 & \ \ -12.96 & \ \ -13.02\\ |
| 422 |
|
\end{tabular} |
| 428 |
|
\section{Conclusions} |
| 429 |
|
|
| 430 |
|
The free energy for proton ordered variants of hexagonal and cubic ice |
| 431 |
< |
$I$, ice B, and recently discovered Ice-{\it i} where calculated under |
| 431 |
> |
$I$, ice B, and recently discovered Ice-{\it i} were calculated under |
| 432 |
|
standard conditions for several common water models via thermodynamic |
| 433 |
|
integration. All the water models studied show Ice-{\it i} to be the |
| 434 |
|
minimum free energy crystal structure in the with a 9 \AA\ switching |
| 437 |
|
solid phase at 1 atm is Ice-{\it i}, not ice $I_h$. The effect of |
| 438 |
|
interaction truncation was investigated through variation of the |
| 439 |
|
cutoff radius, use of a reaction field parameterized model, and |
| 440 |
< |
estimation of the results in the presence of the Ewald summation |
| 441 |
< |
correction. Interaction truncation has a significant effect on the |
| 440 |
> |
estimation of the results in the presence of the Ewald |
| 441 |
> |
summation. Interaction truncation has a significant effect on the |
| 442 |
|
computed free energy values, and may significantly alter the free |
| 443 |
|
energy landscape for the more complex multipoint water models. Despite |
| 444 |
|
these effects, these results show Ice-{\it i} to be an important ice |
| 446 |
|
|
| 447 |
|
Due to this relative stability of Ice-{\it i} in all manner of |
| 448 |
|
investigated simulation examples, the question arises as to possible |
| 449 |
< |
experimental observation of this polymorph. The rather extensive past |
| 449 |
> |
experimental observation of this polymorph. The rather extensive past |
| 450 |
|
and current experimental investigation of water in the low pressure |
| 451 |
< |
regime leads the authors to be hesitant in ascribing relevance outside |
| 452 |
< |
of computational models, hence the descriptive name presented. That |
| 453 |
< |
being said, there are certain experimental conditions that would |
| 454 |
< |
provide the most ideal situation for possible observation. These |
| 455 |
< |
include the negative pressure or stretched solid regime, small |
| 456 |
< |
clusters in vacuum deposition environments, and in clathrate |
| 457 |
< |
structures involving small non-polar molecules. |
| 451 |
> |
regime makes us hesitant to ascribe any relevance of this work outside |
| 452 |
> |
of the simulation community. It is for this reason that we chose a |
| 453 |
> |
name for this polymorph which involves an imaginary quantity. That |
| 454 |
> |
said, there are certain experimental conditions that would provide the |
| 455 |
> |
most ideal situation for possible observation. These include the |
| 456 |
> |
negative pressure or stretched solid regime, small clusters in vacuum |
| 457 |
> |
deposition environments, and in clathrate structures involving small |
| 458 |
> |
non-polar molecules. Fig. \ref{fig:sofkgofr} contains our predictions |
| 459 |
> |
of both the pair distribution function ($g_{OO}(r)$) and the structure |
| 460 |
> |
factor ($S(\vec{q})$ for this polymorph at a temperature of 77K. We |
| 461 |
> |
will leave it to our experimental colleagues to determine whether this |
| 462 |
> |
ice polymorph should really be called Ice-{\it i} or if it should be |
| 463 |
> |
promoted to Ice-0. |
| 464 |
|
|
| 465 |
|
\section{Acknowledgments} |
| 466 |
|
Support for this project was provided by the National Science |
