Water–water and water–ion potential functions including terms for many body effects

We have developed potential functions for water–water and water–ion interactions that include terms to model nonadditive or many body effects. Using these potential functions, we have obtained good agreement with experimental results for gas phase and condensed phase water calculations and excellent results for gas phase ion hydration enthalpies. Some gas phase ion–water cluster display unusual geometries such as the sodium complex with six waters, where four water molecules bind directly to sodium and the two remaining water molecules form hydrogen bonds with the first four water molecules. The chloride complex with four waters is also interesting in that all four waters cluster on the same side of the anion and form weak hydrogen bonds with each other in addition to the linear hydrogen bond each forms with the anion. We have also used the potentials to estimate solvation enthalpies and coordination numbers for several ionic species Na+, K+, Mg++, and Cl−, again obtaining good overall agreement with experimental data. We feel these results demonstrate the superiority of many body potential functions over pair potentials for some applications problems.