Gibbs ensemble simulations of vapor/liquid equilibrium using the flexible RWK2 water potential

Gibbs ensemble Monte Carlo simulations of water vapor/liquid equilibrium (VLE) using the flexible fixed charge RWK2 water-water potential are reported. The equilibrium densities, saturation pressures, and critical parameters calculated with this potential are in better agreement with experimental data than are values obtained from the SPC, MSPC/E, TIP4P, and TIP5P rigid fixed charge potentials as well as from polarizable interactions, such as those of the PPC, KJ, SPC/P, TIP4P/P, and SPCDP potentials. The agreement between predicted and experimental phase coexistence phase density versus temperature variation in the critical region is similar to that obtained from the SPC/E and the polarizable SPC-pol-1 potentials. However, the variation of the saturation pressure and heat of vaporization with temperature and the critical pressure predicted by the RWK2 potential are closer to experiment than predictions from SPC/E interaction and similar (for lower temperatures) to those of the MSPC/E potential. The water structure predictions of the RWK2 potential are substantially better than those from the SPC-pol-1 interaction. VLE data are not used to parametrize the RWK2 potential. Nevertheless, predictions for coexistence temperature versus phase density and saturation pressure versus temperature are similar to those of the recent potential of Errington and Panagiotopoulos (EP potential), which is adjusted to reproduce the VLE behaviour of water. MD simulations using the RWK2 potential show much better agreement with the room-temperature liquid structure and second virial coefficient data than does the EP potential. Fixing the bond lengths and HOH angle in the RWK2 interaction at their isolated molecule minimum energy values leads to significant deterioration of the VLE predictions obtained.