A family of primitive models of water: three-, four and five-site models

Abstract

A family of primitive models of water, which differ from one another in the number and location of interaction sites, is introduced and their properties examined by Monte Carlo simulations. In addition to the existing symmetric 5-site model, which has its origin in the ST2 potential, asymmetric 3- and 4-site descendants of TIPS potentials are introduced along with an extended 5-site model which incorporates a short-range repulsion between the like sites. The structure of the fluids defined by the primitive models has been investigated in detail by computing site-site correlation functions, both at high and low densities, and the angular distribution of particles engaged in hydrogen bonding. For completeness, the thermodynamic properties have also been computed. It transpires that the extended 5-site model, due to its enlarged range of the hydrogen bond interaction, clearly is much better than all the other models. It is able to reproduce even semi-quantitatively the structure of real water, and thus seems well suited to all potential applications involving water, including perturbation theories using the extended model as a reference fluid.