Natural Energy Decomposition Analysis:  Explicit Evaluation of Electrostatic and Polarization Effects with Application to Aqueous Clusters of Alkali Metal Cations and Neutrals

Natural energy decomposition analysis (NEDA) is extended to calculate electrostatic and polarization contributions. NEDA is a Hartree−Fock-based approach that facilitates the calculation of the electrostatic, polarization, charge transfer, exchange, and deformation components of intermolecular interactions. Analysis of the aqueous clusters of the alkali metal cations, M+(H2O)n (n = 1−4), demonstrates the reasonable behavior of the NEDA components and dipole moments with changes in geometry and coordination. In general, the electrostatic and polarization components behave as anticipated from a classical treatment based on point charges, dipoles, and polarizabilities. Extended basis set applications demonstrate the high numerical stability of the method whereas comparison calculations with the Morokuma analysis show contrastingly poor basis set convergence. The popular 6-31+G* basis set yields a binding energy for Li+(H2O) in good agreement with the estimated complete basis set (CBS) limit. However, comparison of the 6-31+G* and CBS NEDA results reveals that this agreement is fortuitous, relying on a cancellation of errors that stem from the inability of this basis set to accurately describe the dipole moment and polarizability of water. Representative calculations are also presented for open-shell clusters Na(H2O)n (n = 1−4) at the unrestricted-Hartree−Fock level.