Problems with Some Current Water Models for Close Pair Interactions That Are Not Near the Minimum Energy

The ability of an empirical, polarizable model of water to predict a thermal ensemble of molecular configurations at ambient conditions was examined using first-principle quantum mechanics. The empirical model of water selected for this evaluation was the TTM2-F model. The quantum mechanical methodology selected was the second-order Møller-Plesset model (MP2). Only pairwise interaction energies were considered. Significant deviations from the empirical model were found. Similar results were found for ad-hoc comparisons with several other common water models including the TIP3P, TIP4P, TIP4P-FQ, TIP5P, TTM2.1-F, TTM2.2-F, TTM3-F, and POL5/QZ potential models. Our results show that spatially close dimer configurations with interaction energies notably above the potential well minimum (but are still thermally accessible at ambient conditions) are the source of the largest deviations. To assist others in future water model parametrizations we report the MP2 near complete basis set limit energies for 840 water dimer configurations sampled from an approximate thermal ensemble at ambient conditions.