A molecular dynamics study of sub- and supercritical water using a polarizable potential model

Abstract

A series of molecular dynamics calculations for water has been carried out along an isochore at 1 g/cm3 and an isotherm at 600 K in order to examine microscopic properties of water in the sub- and supercritical states. A polarizable potential model proposed by Dang (RPOL model) was employed to take into account the state dependence of intermolecular interaction. Along the isochore, fluid structure changes from tetrahedral icelike structure at room temperature to simple-liquidlike one at high temperatures. Orientational correlation between a tagged molecule and its neighbors is reduced substantially with increasing temperature, though hydrogen bonds between two molecules persist even at 600 K. As temperature increases, the number of the hydrogen bonds per molecule decreases monotonically from 3.2 at 280 K to 1.9 at 600 K. The activation barrier for diffusion at 600 K is about half as large as that at room temperature. A collective polarization relaxation loses collective character above the temperature where the structural change occurs. Along the isotherm, on the other hand, the long-ranged tail of radial distribution functions was observed near the critical density ρc. Ornstein-Zernike behavior, however, was not found owing to the present small system. The number of hydrogen bonds decreases almost linearly as a function of the density from 1.9 at 1 g/cm3 to 0 in the gas limit. However, the hydrogen bonds were still found near the critical density. At densities below ρc, density dependence of the diffusion coefficients are qualitatively described by the simple kinetic theory for gases. At higher densities, the diffusion coefficients deviate from the prediction by the kinetic theory. Rotational correlation function at low density has the form similar to free rotors, while at high densities, the rotational relaxation may be described by rotational diffusion. It indicates that the rotational dynamics changes continuously around the critical density from a gaslike one to a liquidlike one.