A MOLECULAR-DYNAMICS STUDY OF SUBCRITICAL AND SUPERCRITICAL WATER USING A POLARIZABLE POTENTIAL MODEL

A series of molecular dynamics calculations for water has been carried out along an isochore at 1 g/cm(3) and an isotherm at 600 K in order to examine microscopic properties of water in the sub- and supercritic al states. A polarizable potential model proposed by Dang (RPOL model) was employed to take into account the state dependence of intermolecu lar interaction. Along the isochore, fluid structure changes from tetr ahedral icelike structure at room temperature to simple-liquidlike one at high temperatures. Orientational correlation between a tagged mole cule and its neighbors is reduced substantially with increasing temper ature, though hydrogen bonds between two molecules persist even at 600 K. As temperature increases, the number of the hydrogen bonds per mol ecule decreases monotonically from 3.2 at 280 K to 1.9 at 600 K. The a ctivation barrier for diffusion at 600 K is about half as large as tha t at room temperature. A collective polarization relaxation loses coll ective character above the temperature where the structural change occ urs. Along the isotherm, on the other hand, the long-ranged tail of ra dial distribution functions was observed near the critical density rho (c). Ornstein-Zernike behavior, however, was not found owing to the pr esent small system. The number of hydrogen bonds decreases almost line arly as a function of the density from 1.9 at 1 g/cm(3) to 0 in the ga s limit. However, the hydrogen bonds were still found near the critica l density. At densities below rho(c), density dependence of the diffus ion coefficients are qualitatively described by the simple kinetic the ory for gases. At higher densities, the diffusion coefficients deviate from the prediction by the kinetic theory. Rotational correlation fun ction at low density has the form similar to free rotors, while at hig h densities, the rotational relaxation may be described by rotational diffusion. It indicates that the rotational dynamics changes continuou sly around the critical density from a gaslike one to a liquidlike one . (C) 1998 American Institute of Physics.