A novel approach for designing simple point charge models for liquid water with three interaction sites

A simultaneous improvement of the diffusion and dielectric properties of the simple point charge (SPC) model for liquid water appears to be very difficult with conventional reparametrization of the commonly used Lennard-Jones and Coulomb interaction functions and without including a self-energy correction in the effective pair-potential as is done in the SPC/E model. Here, a different approach to circumvent this problem is presented. A short-range interaction term, which corrects the oxygen-oxygen energy at small distances by small amounts of energy, was introduced in the nonbonded interaction function. This additional force-field term allows to derive new parameter sets for SPC-like water models that yield better agreement with experimental data on liquid water. Based on previous investigations of the force-field parameter dependence of the water properties of SPC-like models, the necessary parameter changes to obtain a lower diffusion coefficient and a larger dielectric permittivity were specified and accordingly six new models were developed. They all represent an improvement over SPC in terms of structural and diffusional properties, four of them show better dielectric properties also. One model, SPC/S, has been characterized in more detail, and represents most properties of liquid water better than SPC while avoiding the larger discrepancies with experimental values regarding density, thermal compressibility, energy, and free energy of the SPC/E model. We conclude that the use of a simple, short-ranged additional oxygen-oxygen interaction term makes a simultaneous improvement of the diffusion coefficient and the dielectric properties of water feasible.