Fugacity Coefficients of Saturated Water from Molecular Simulation

We apply isothermal-isobaric Monte Carlo molecular simulation to measure the vapor-phase fugacity coefficients and equation of state of three fixed-charge models of water: SPC/E, MSPC/E, and TIP3P. State conditions correspond to the experimental vapor-liquid saturation pressures for temperatures between 292.65 and 386.85 K. The results are compared to the experimental data for water vapor. We also examine properties as given by the application of the virial equation of state, using values of the second virial coefficients computed by us or that have been previously reported (in some cases up to third virial coefficients) for the models simulated here. The models show significant deviation from ideal-gas behavior, considerably more than seen experimentally, and a second-order virial treatment does not fully characterize the behavior. This deviation from experiment is consistent with the expected failings of fixed-charge models developed for liquid phases, when applied to the vapor. A virial-equation treatment of a fluctuating-charge model (TIP4P/fq) and the polarizable point charge model (PPC) indicates that the vapor-phase properties of these models will be much more in line with experimental fugacities and densities along the coexistence line.