Utility of chemical computations in predicting solution free energies of metal ions

Abstract

Here, we study quasi-chemical theory (QCT) for the free energies of divalent alkaline earth ions (Ba, Sr, Ca, Mg) in water, emphasizing that: (a) interactions between metal ions and proximal water molecules are as strong as traditional chemical effects; (b) QCT builds directly from accessible electronic structure calculations but rests on fully elaborated molecular statistical thermodynamics; (c) QCT offers choices of convenience in identifying coordination numbers for analysis. We investigate utilisation of direct QCT with inner-shell conditioning , alternative to the traditional conditioning motivated by a generalised van der Waals view. The alternative works well: deleterious non-Gaussian effects of van der Waals repulsive interactions are not serious, and the alternative conditioning improves the convenience of QCT calculations. Comparison between ab initio and force field molecular dynamics (AIMD and FFMD) with standard models suggests that FFMD likely exaggerates the anharmonicity in the thermal motion of inner-shell ion-water clusters. Together with the general encouraging support for the harmonic approximations implied by the conditioning, that observation helps explain the remarkable success of the cluster-based QCT solution free energies, which do not require assessment of all inner-shell occupancies by simulation.