LIQUID WATER WITH AN ABINITIO POTENTIAL - X-RAY AND NEUTRON-SCATTERING FROM 238-K TO 368-K

Molecular dynamics simulations with the "Nieser-Corongiu-Clementi flexible water molecules" (NCC-vib) ab initio potential are reported at a number of temperatures ranging from 242 to 361 K for H2O and from 238 to 368 K for D2O. Each simulation has been carried out, after equilibration, for 12 ps; the computed trajectories have been used to study structural properties as a function of the temperature. Water geometry in the liquid, pair correlation functions, coordination numbers, x-ray and neutron scattering structure functions are compared with available experiments. The overall agreement is quite satisfactory, validating the reliability of the NCC-vib interaction potential. Concerning the pair correlation [g(alpha-beta)(r,T)] and the neutron scattering structure functions, our simulation validates more Soper et al. [Chem. Phys. 107 (1986)] than Thiessen et al. [J. Chem. Phys. 77 (1982)] data. Plots of the DELTA-g(alpha-beta)(r,DELTA-T) and of the temperature variations in the coordination numbers are favorably compared with the experimental findings by Bosio et al. [Phys. Rev. A 27 (1983)]. Temperature dependent shifts in peak position and intensity of x-ray and neutron structural functions are in excellent agreement with the experimental finding of Bosio et al. [Phys. Rev. A 27 (1983)] and Chen et al. [Adv. Chem. Phys. 64 (1988)]. These simulations point out that for liquid water, in the temperature range approximately 238 to 368 K, at the density of 0.997 g/cm3, the N(OO) oxygen coordination number grows, with a well defined transition, from 4 to 5 while the N(OH) decreases from 5 to 4.