A theoretical study of the water dimer interaction

We have performed a study of the water dimer interaction using larger basis sets and higher levels of theory than have been previously applied to this system. For the minimum geometry we have used spdf basis sets containing up to 212 orbitals. Our most accurate SCF interaction energy for the minimum is −3.73±0.05 kcal/mol. We have shown that this energy can be reproduced to within 0.1 kcal/mol using much smaller basis sets containing proper (diffuse) exponents. Accounting for the basis set superposition error is shown to be essential. We computed the dispersion energy with neglect of the intramolecular correlation using basis sets of various sizes. The best value obtained in a large spdf basis set with exponents which optimize this quantity is −1.93 kcal/mol and it is expected to be accurate to 0.1 kcal/mol or better. Using some of these basis sets we have performed supermolecular many-body perturbation theory (MBPT) and coupled-cluster (CC) calculations including triple excitations. We have shown that if the correlated supermolecular interaction energies are not corrected for the basis set superposition error, the correlated part of the interaction energy varies widely with the basis set. In contrast, the corrected values converge smoothly. On the basis of the dispersion energy and the MBPT/CC results we predict the correlated part of the interaction energy to be −1.0±0.3 kcal/mol, which leads to the total interaction energy of −4.7±0.35 kcal/mol. Thus, our work favors the lower limit of the experimentally predicted interaction energy of −5.4±0.7 kcal/mol.