Development of Polarizable Water Force Fields for Phase Equilibrium Calculations

Recent simulation studies for four dipole-polarizable and for two fluctuating-charge water force fields have demonstrated that none of the force fields studied is capable of yielding a satisfactory description of the vapor-liquid coexistence curve from room temperature to the critical region. The performance of these polarizable force fields can be improved dramatically by introducing an additional coupling between the Lennard-Jones interaction parameters for a pair of oxygen sites and their partial charges (electronic configuration). Two different types of water models are presented which are based on either the three-site simple point charge (SPC) or the four-site transferable intermolecular potential 4 point (TIP4P) water representations. Adiabatic nuclear and electronic sampling Monte Carlo (ANES-MC) simulations in the Gibbs, isobaric-isothermal, and canonical ensembles were performed to calculate vapor-liquid coexistence curves, to determine the temperatures of maximum liquid density, and to evaluate dielectric constants along the coexistence line, respectively. The new SPC-pol-1 force field yields significantly better agreement with the experiment for the saturated vapor and liquid densities, the heats of vaporization, and the liquid-phase dielectric constants than the fixed-charge SPC, SPC/E (simple point charge/extended), TIP4P, and Errington/ Panagiotopoulos (EP) force fields or than any other polarizable force field previously tested. However, the representation of the liquid water structure at ambient conditions is less satisfactory for the SPC-pol force fields. In contrast, the TIP4P-pol force fields produce much better low-temperature liquid structures and, in particular, a density maximum close to T = 277 K, but their performance for the vapor-liquid equilibria in the near-critical region is less satisfactory. Finally, it is important to note that the SPC-pol-1 force field yields an average molecular dipole moment of 2.5 D for the liquid phase at ambient conditions that is substantially smaller than the value of 2.7 D obtained for its minimum-energy hexamer cluster.