We determined the freezing point of eight molecular models of water. All models use the charge-on-spring (COS) method to express polarization. The studied models were the COSG2, COSG3 H. Yu and W. F. van Gunsteren, J. Chem. Phys. 121, 9549 (2004)10.1063/1.1805516, the COSB2 H. Yu, T. Hansson, and W. F. van Gunsteren, J. Chem. Phys. 118, 221 (2003)10.1063/1.1523915, the SWM4-DP G. Lamoureux, A. D. MacKerell, Jr., and B. Roux, J. Chem. Phys. 119, 5185 (2003)10.1063/1.1598191, the SWM4-NDP G. Lamoureux, E. Harder, I. V. Vorobyov, B. Roux, and A. D. MacKerell, Jr., Chem. Phys. Lett. 418, 245 (2006)10.1016/j.cplett.2005.10.135, 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)10.1063/1. 3692602 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)10.1063/1.3670962, 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, 084506 (2012)10.1063/1.4746419. We used the thermodynamic integration (TI) and the Gibbs-Helmholtz equation to determine the equality of the free energy for liquid water and hexagonal ice (Ih) at 1 bar. We used the TIP4P and the SPCE models as reference systems of the TI. The studied models severely underestimate the experimental melting point of ice Ih. The calculated freezing points of the models are the following: COSG2, 215 K; COSG3, 149 K; SWM4-DP, 186 K; BKd1, 207 K; BKd2, 213 K; BKd3, 233 K. The freezing temperature of the SWM4-NDP system is certainly below 120 K. It might even be that the water phase is more stable than the ice Ih at 1 bar in the full temperature range. The COSB2 model melts below 100 K. The best result was obtained for the BKd3 model which indicates that correct description of the (T-ρ) curve improves the estimation of the freezing point. We also determined and compared the densities of high-pressure polymorphs of ice for these models.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry