Computational determination of equilibrium geometry and dissociation energy of the water dimer

The equilibrium geometry and dissociation energy of the water dimer have been determined as accurately as technically possible. Various quantum chemical methods and high-quality basis sets have been applied—that is, at the level of a nearly complete basis—and both the intermolecular separation and the deformation of the donor and acceptor molecules have been optimized at the level of CCSD(T) theory (coupled-cluster theory with singles and doubles excitations plus a perturbation correction for connected triples). It is found at the CCSD(T) level that the monomer deformation in the dimer amounts to 86% of the deformation computed at the MP2 level (second-order Møller-Plesset perturbation theory) and that the core/valence electron correlation effects at the CCSD(T) level amount to 80% of the same effects at the MP2 level. The equilibrium O···O distance is determined as Re=291.2±0.5 pm and the equilibrium dissociation energy as De=21.0±0.2 kJ mol−1, with respect to dissociation into two isolated water molecules at equilibrium. Accounting for zero-point vibrational energy, the theoretical prediction for the dissociation energy becomes D0=13.8±0.4 kJ mol−1, a result which is open to direct experimental verification.