Many-body effects in ion-water interactions: Fe³⁺ in water

Two-, three-, and four-body interaction energies are calculated for tetrahedral, octahedral, and cubic arrangements of H₂O molecules around an Fe³⁺ ion using ab initio molecular orbital theory. The results show a slow convergence of the interaction terms and the importance of many-body effects, which may account for the incorrect coordination number obtained using ab initio pair potentials in computer simulations of Fe³⁺ and other multivalent ions in water.