Extending the polarizable continuum model to effective ab initio pair potentials in multicomponent solutions: A test on calcium–water and calcium–ammonia potentials

The use of the polarizable continuum model to develop ab initio effective pair potentials is extended to multicomponent solutions. The methodology takes into account nonadditivity effects on pair interactions computing wave functions perturbed by the solvent. $Ca2+$–water and $Ca2+$–ammonia potentials suitable for aqueous ammonia solutions are presented. These effective ab initio pair potentials present smaller binding energies with respect to strictly ab initio two-body potentials. The reduction is higher in $Ca2+$–ammonia (28%) than in $Ca2+$–water (22%) and brings to a small gap the difference between the binding energies of the two ligands with $Ca2+$ when solvent effects are considered. As a first test, metal-ligand clusters of different size and composition have been studied. The comparison with restricted Hartree–Fock ab initio calculations shows good agreement for the largest clusters considered. Results confirm that the presented methodology, based on the polarizable continuum model, describes in a proper way the interactions in the condensed phase, where the ion completes its coordination sphere. The cluster results also show that ammonia can displace water in the first ion coordination with a tendency to change the coordination number from 8 to 9 when the ion is fully surrounded by the former, the ninth ammonia molecule being positioned in an intermediate situation between the first and the second coordination shells.