Accurate molecular geometries of the protonated water dimer

Abstract

The equilibrium geometry of the protonated water dimer, H₅O₂⁺, was studied using Møller–Plesset perturbation theory and coupled-cluster theory. Constrained geometry optimizations were carried out for the C₂ and C_s symmetric structures within the counterpoise framework and near the limit of a complete basis set. In the constrained optimization, the degrees of freedom of the complex are reduced to an intrafragmental distortion and an interfragmental coordinate, making the procedure tractable for large basis sets and explicitly correlated linear r₁₂ methods. The energy of the stationary point of C₂ symmetry was found to be 1.2 kJ mol⁻¹ below the energy of the C_s structure.