Microstructure of ambient and supercritical water. Direct comparison between simulation and neutron scattering experiments

Molecular dynamics simulations of SPC, SPC/E, TIP4P, and ST2 water models are performed at ambient and two supercritical conditions to make a direct comparison with recent microstructural data obtained by neutron diffraction with isotopic substitution (NDIS) experiments. The models generally fail to accurately predict details of the NDIS results at supercritical conditions, even though they are somewhat successful at ambient conditions. The failure is not as pronounced as that expected by Postorino et al. (Lett. Nature 1993, 366, 668-670) because of an unusual density dependence in the structure by two of the models. We also evaluate a model for supercritical water denoted SPCG, a modification of the SPC and SPC/E models, in which the dipole moment is reduced to the bare dipole moment of water. For this model, the predicted structure at supercritical conditions is in much better agreement with experiment. A geometric definition of hydrogen bonding is used to gain insight into the angular dependence of the H ... O pair distribution function g(OH)(r,omega). The simulation results for the five models indicate a strong orientational dependence for the g(OH)(r,omega) along the H-bonding orientations, with an approximately constant relative strength from ambient to supercritical conditions, suggesting that the angle-averaged radial distribution function, g(OH)(r), and its volume integral over the first solvation shell, n(OH)(r), may not in themselves be good measures of the strength of the H-bonding.