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

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Abstract
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.

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