HYDRATION SHELL STRUCTURE OF THE CALCIUM-ION FROM SIMULATIONS WITH AB-INITIO EFFECTIVE PAIR POTENTIALS

Computer simulations of dilute aqueous solutions of Ca2+ have been per formed with a strictly two-body ab initio ion-water potential and an e ffective ab initio pair potential. The latter includes many-body effec ts in an average way relying on a continuum polarizable model for the solvent. With the two-body potential, the structure of the first hydra tion shell of Ca2+ turns out to be composed of nine molecules in an ar rangement of approximate symmetry D3h (hydration number=9.1). This str ucture is also observed with the effective potential, but it alternate s with another that consists of eight molecules in an antiprism config uration, so that the average hydration number is 8.6.