A systematic ab initio study of the water dimer in hierarchies of basis sets and correlation models

A systematic, high-level ab initio investigation of the water dimer has been performed. The oxygen-oxygen bond distance has been estimated to be around 2.90 Angstrom, about 0.05 Angstrom shorter than the experimentally estimated distance, challenging the accuracy of the latter. The interaction energy has been obtained at -5.0 +/- 0.1 kcal/mol, which compares favourably with the experimentally estimated value of -5.4 +/- 0.7 kcal/mol. The importance of employing basis sets that include diffuse functions in correlated calculations on hydrogen-bonded systems is confirmed. In correlated calculations on the water dimer and the hydrogen fluoride dimer. the counterpoise-corrected interaction energies converge considerably slower towards the basis set limit than do the uncorrected energies, provided that the correlation-consistent basis sets are augmented with diffuse functions.