A fixed point charge model for water optimized to the vapor-liquid coexistence properties

A new fixed-point charge potential model for water has been developed, targeting the accurate prediction of the vapor-liquid coexistence properties over a broad temperature range. The model consists of a Buckingham exponential-6 group at the oxygen center and a set of three point charges. The new model was designed primarily to reproduce coexisting vapor and liquid densities, vapor pressures, and critical parameters and secondarily the liquid structure of water at ambient conditions. To obtain an optimum set of intermolecular potential parameters, the Gibbs ensemble and histogram reweighting grand canonical Monte Carlo methods were used to test a large number of models that covered a wide range of parameter space. Recently developed Hamiltonian scaling grand canonical Monte Carlo techniques enabled us to move through parameter space in an efficient manner. Properties of the new model were determined over the temperature range of the liquid. Results were compared to the SPC, SPC/E, and MSPC/E models and to experimental data. The new model was found to be better than existing models for the coexistence densities, vapor pressures, critical parameters, and the second virial coefficient. It is inferior to the SPC/E and MSPC/E models in reproducing the dielectric constant. The oxygen-hydrogen and hydrogen-hydrogen radial distributions are in good agreement with experimental data; however, the second shell of the oxygen-oxygen radial distribution of the new model does not have the correct form.