Free energies and structures of hydrated cations, based on effective pair potentials Original Research Article

We present a method, based on a continuum representation of the solvent, to compute ab initio effective interaction potentials for solvated pairs. Such potentials take into account many-body effects, thus overcoming the non-additivity errors affecting uncorrected pair potentials. We apply the method to cation-water interactions, for a variety of cations: Li+, Be2+, Mg2+, Ca2+, Ni2+, Zn2+ and Al3+. The potentials thus obtained are suitable for simulations of ionic solutions or clusters of water molecules surrounding a cation. We exploit them to compute hydration free energies ΔGhyd of cations, with the constraint that the first solvation shell contains a given number of water molecules. This enables us to find the thermodynamically most stable solvation number. The effective potential results compare well with experimental values of ΔGhyd and with full ab initio calculations on the [M(H2O)n]q+ complexes.