Nature of Alkali Ion-Water Interactions: Insights from Many-Body Representations and Density Functional Theory. II

Interaction energies of alkali ion-water dimers, M⁺(H₂O), and trimers, M⁺(H₂O)₂, with M = Li, Na, K, Rb, and Cs, are investigated using various many-body potential energy functions and exchange-correlation functionals selected across the hierarchy of density functional theory approximations. Analysis of interaction energy decompositions indicates that close-range interactions such as Pauli repulsion, charge transfer, and charge penetration must be captured in order to reproduce accurate interaction energies. In particular, it is found that simple classical polarizable models must be supplemented with dedicated terms which account for these close-range interactions in order to achieve chemical accuracy. It is also found that the exchange-correlation functionals mostly differ from each other in their Pauli repulsion + dispersion energies and, hence, benefit from the inclusion of nonlocal terms such as Hartree-Fock exchange and dependence on the electronic kinetic energy density in order to reproduce the interactions that contribute to this term, namely, Pauli repulsion and intermediate-range dispersion. As a continuation of the analysis performed in J. Chem. Theory Comput. 2019, 15, 2983, 10.1021/acs.jctc.9b00064, we make comparisons between findings for alkali ion-water interactions with those for halide-water interactions.