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 performed with a strictly two-body ab initio ion-water potential and an effective ab initio pair potential. The latter includes many-body effects in an average way relying on a continuum polarizable model for the solvent. With the two-body potential, the structure of the first hydration shell of Ca2+ turns out to be composed of nine molecules in an arrangement of approximate symmetry D3h (hydration number=9.1). This structure is also observed with the effective potential, but it alternates with another that consists of eight molecules in an antiprism configuration, so that the average hydration number is 8.6.