A FAMILY OF PRIMITIVE MODELS OF WATER - 3-SITE, 4-SITE AND 5-SITE MODELS

A family of primitive models of water, which differ from one another i n the number and location of interaction sites, is introduced and thei r properties examined by Monte Carlo simulations. In addition to the e xisting symmetric 5-site model, which has its origin in the ST2 potent ial, asymmetric 3- and 4-site descendants of TIPS potentials are intro duced along with an extended 5-site model which incorporates a short-r ange repulsion between the like sites. The structure of the fluids def ined by the primitive models has been investigated in detail by comput ing site-site correlation functions, both at high and low densities, a nd 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 ra nge 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 th e extended model as a reference fluid.