Application of systematic sequences of wave functions to the water dimer

A systematic series of calculations encompassing a wide range of basis sets and correlated methods has been used to estimate the complete basis set, full CI hydrogen bond strength in the water dimer system. The largest basis set included up through h polarization functions on oxygen and g functions on hydrogen. The complete basis set limit for the self-consistent-field (SCF) interaction energy is estimated to be −3.55 kcal/mol with an accompanying correlation contribution of ∼−1.5 kcal/mol. This leads to an interaction energy of −5.1 kcal/mol, exclusive of vibrational zero-point considerations, and is in good agreement with experimental measurements of −5.4±0.7 kcal/mol. Inclusion of an approximate adjustment for the basis set superposition error via the Boys/Bernardi counterpoise correction was found to substantially improve agreement with ΔE∞, our estimate of the complete basis set interaction energy, at the both the SCF and correlated levels for basis sets that were lacking in sufficient near-valence diffuse functions. For diffuse-function-augmented basis sets, application of the CP correction was found to sometimes worsen agreement with ΔE∞.