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 ha s been performed. The oxygen-oxygen bond distance has been estimated t o be around 2.90 Angstrom, about 0.05 Angstrom shorter than the experi mentally estimated distance, challenging the accuracy of the latter. T he interaction energy has been obtained at -5.0 +/- 0.1 kcal/mol, whic h 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 system s is confirmed. In correlated calculations on the water dimer and the hydrogen fluoride dimer. the counterpoise-corrected interaction energi es converge considerably slower towards the basis set limit than do th e uncorrected energies, provided that the correlation-consistent basis sets are augmented with diffuse functions.