Phase equilibria in extended simple point charge ice-water systems

The characteristics of the solid/liquid transition for a modified Simple Point Charge model of water have been determined using free energy calculations supported by nonequilibrium Molecular Dynamics (NEMD) simulations. We have considered the behavior of liquid water and of a variety of ice phases. Unlike real water, the stable crystalline phase at 1 bar is not hexagonal ice I, but a denser new ice phase. The melting point of this ice was found to be near 295 K. The lower-density ices, Ih and Ic, are less stable than water down to the glass transition temperature. The conclusions are supported by NEMD simulations of the behavior of the planar crystal–liquid interface for these different cases. The first report of the growth of ice from water using molecular simulation is shown here. The influence of the components of the intermolecular potential on the stability of the ice polymorphs is investigated. It is found that, for ice I to be the stable phase, the Lennard–Jones attractive part should be reduced, and the potential switching function should be used at longer distances. Properties of the supercooled liquid water are also presented.