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.