Global minima and energetics of Li+(H2O)n and Ca2+(H2O)n clusters for n ⩽ 20

We present candidate structures for the global potential energy minima of X(H2O)n (X = Li+, Ca2+) clusters with n ⩽ 20. The TIP4P rigid body water-water potential is employed, together with cation-water interactions that are represented as a sum of Coulomb and Lennard-Jones terms. We find that in the Li+ global minima the cation tends to be located on the cluster surface with a preferred coordination number of four water molecules in tetrahedral positions. In contrast, for Ca2+ clusters the cation occupies an interior site surrounded by eight water molecules in a square anti-prismatic arrangement.