A very simple method for the extraction of effective interaction potentials from ab initio calculations was proposed (Periole et al. J. Phys. Chem. 1997, 101, 5018), and simple two-body cation-water interaction potentials were derived for several cations, Li+, Na+, K+, Be2+, Mg2+, and Ca2+, using two facts: first, water molecules in the close vicinity of cations are strongly structured and present a constrained orientation towards the ion
second, at larger distances the ion-water interaction is mainly electrostatic. In the present work, an extension to Rb+ and Sr2+ and some refinements of this method are presented. In particular, we explore the most adequate way of including the nonadditivity and polarization effects that arise from the ion-water-water and water-water interactions. The potentials obtained with the new extraction methods are compared with the empirical potentials of Aqvist (Aqvist, J. J. Phys. Chem. 1990, 94, 8021) that were adjusted to reproduce experimental data. Those obtained with the exploration-TIE method are also tested by performing molecular dynamics simulations of the various cation-water systems and the results are found to be in good agreement with experimental data. In particular, they yield cation hydration free energy differences (Delta G values) that are, in general, in good accordance with experimental figures. This tatter method is ideally suited and easy to apply to obtain effective interaction potentials for molecular systems with restricted geometric conditions that appear in numerical simulations, either Monte Carlo or molecular dynamics.
