Nonadditivity of ab initio pair potentials for molecular dynamics of multivalent transition metal ions in water

Ab initio molecular orbital pair potentials for the interaction of Fe ²⁺ and Fe³⁺ ions with H₂O are reported. Molecular dynamics calculations of the static structure of the solvation shell of Fe²⁺ and Fe³⁺ in water using the ab initio pair potentials gives physically incorrect results, i.e., the coordination numbers are eight instead of six as observed experimentally. This problem has also been encountered by other workers for divalent transition metal ions in water. By computing three-body energies from the interaction of two water molecules with the cations, we show that the origin of the problem is most likely in the assumption of the additivity of the pair potentials, i.e., neglect of many-body forces. Empirical potentials are reported which take approximate account of the three-body forces and give coordination numbers of six for both Fe²⁺ and Fe³⁺ in water.