NEW EFFECTIVE POTENTIALS EXTRACTION METHOD FOR THE INTERACTION BETWEEN CATIONS AND WATER

A very simple method for the extraction of effective interaction poten tials 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 vicin ity of cations are strongly structured and present a constrained orien tation towards the ion; second, at larger distances the ion-water inte raction is mainly electrostatic. In the present work, an extension to Rb+ and Sr2+ and some refinements of this method are presented. In par ticular, we explore the most adequate way of including the nonadditivi ty and polarization effects that arise from the ion-water-water and wa ter-water interactions. The potentials obtained with the new extractio n methods are compared with the empirical potentials of Aqvist (Aqvist , J. J. Phys. Chem. 1990, 94, 8021) that were adjusted to reproduce ex perimental data. Those obtained with the exploration-TIE method are al so 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 fr ee energy differences (Delta G values) that are, in general, in good a ccordance with experimental figures. This tatter method is ideally sui ted and easy to apply to obtain effective interaction potentials for m olecular systems with restricted geometric conditions that appear in n umerical simulations, either Monte Carlo or molecular dynamics.