A molecular Ornstein-Zernike study of popular models for water and methanol

Some effective models of water (TIP3P, SPC, SPC/E, TIP4P) and methanol (OPLS, I-Il) sire studied with the help of the molecular Ornstein-Zernike (MOZ) theory using the hypernetted chain (HNC) approximation. The quality of the results obtained within the HNC approximation is discussed by comparison with values from molecular dynamics (MD) simulations. The MOZ-HNC theory yields internal excess energies and dielectric constants which are about 20% smaller than the simulation results found in the literature. The relative trends of the properties observed by simulation for the different interaction models are correctly predicted. In order to calculate the rotational invariant coefficients which define the liquid structure, new MD simulations were carried out. The rotational invariant coefficients derived from the simulation and from the MOZ theory strongly differ. In particular, the center-center distribution functions show that the theory is not able to reproduce the tetrahedral structure of water. In this solvent a comparison of the O-H distribution function indicates that the MOZ theory underestimates the H bonding. The use of a spherically symmetric bridge function in the reference HNC approximation does not lead to an improvement of the MOZ results. The observed defects of the MOZ-HNC approach contrast with the good agreement found for aprotic solvents and are presumably due to the association by H bonds in water and methanol. (C) 1999 American Institute of Physics. [S0021-9606(99)52102-8].