Liquid water with an ab initio potential: X-ray and neutron scattering from 238 to 368 K

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Abstract
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 [gαβ(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 Δgαβ(r,ΔT) 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 ∼238 to 368 K, at the density of 0.997 g/cm3, the NOO oxygen coordination number grows, with a well defined transition, from 4 to 5 while the NOH decreases from 5 to 4.