SCF-CI studies of correlation effects on hydrogen bonding and ion hydration

Previous single-determinant Hartree-Fock studies on the equilibrium structures and stabilities of H₂ O, H₃ O⁺ as well as of the monohydrated ionic systems Li⁺ · H₂O, F^? · H₂O and the hydrogen bonded water dimer, H₂ O · HOH, are extended by large scale configuration interaction calculations including all the possible single and double excitations arising from the canonical set of Hartree-Fock molecular orbitals. The correlation energy effects on the equilibrium geometrical parameters of the systems under consideration are found to be quite small. The contributions of the correlation energy to the total binding energies of the weakly interacting composed systems are obtained to be of the order of 1 kcal/mole, leading to a considerable increase of the hydrogen bond strength in F^? · H₂O and H₂O · HOH and to a small decrease of the binding energy in Li⁺ · H₂ O. The observed strengthening of the hydrogen bonding interaction due to correlation is shown to be partly compensated by the change in the vibrational zero-point energy of the composed systems compared to the non-interacting subsystems. Approximate force constants corresponding to the intersystem vibrations in Li⁺ · H₂O, F^? · H₂ O, and H₂O · HOH are deduced from the calculated potential curve data on the SCF and the CI level of accuracy.