Testing the recent charge-on-spring type polarizable water models. II. Vapor-liquid equilibrium.

We studied the vapor-liquid coexistence region of seven molecular models of water. All models use the charge-on-spring (COS) method to express polarization. The studied models were the COS∕G2, COS∕G3 [H. Yu and W. F. van Gunsteren, J. Chem. Phys. 121, 9549 (2004)], the SWM4-DP [G. Lamoureux, A. D. MacKerell, Jr., and B. Roux, J. Chem. Phys. 119, 5185 (2003)], the SWM4-NDP [G. Lamoureux, E. Harder, I. V. Vorobyov, B. Roux, and A. D. MacKerell, Jr., Chem. Phys. Lett. 418, 245 (2006)], and three versions of our model, the BKd1, BKd2, and BKd3. The BKd1 is the original Gaussian model [P. T. Kiss, M. Darvas, A. Baranyai, and P. Jedlovszky, J. Chem. Phys. 136, 114706 (2012)] with constant polarization and with a simple exponential repulsion. The BKd2 applies field-dependent polarizability [A. Baranyai and P. T. Kiss, J. Chem. Phys. 135, 234110 (2011)], while the BKd3 model has variable size to approximate the temperature-density (T-ρ) curve of water [P. T. Kiss and A. Baranyai, J. Chem. Phys. 137, 194102 (2012)]. We calculated the second virial coefficient, the heat of vaporization, equilibrium vapor pressure, the vapor-liquid coexistence curve, and the surface tension in terms of the temperature. We determined and compared the critical temperatures, densities, and pressures of the models. We concluded that the high temperature slope of the (T-ρ) curve accurately predicts the critical temperature. We found that Gaussian charge distributions have clear advantages over the point charges describing the critical region. It is impossible to describe the vapor-liquid coexistence properties consistently with nonpolarizable models, even if their critical temperature is correct.