Accurate molecular geometries of the protonated water dimer

The equilibrium geometry of the protonated water dimer, H5O2+, was studied using Moller-Plesset perturbation theory and coupled-cluster theory. Constrained geometry optimizations were carried out for the C-2 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 r(12) methods. The energy of the stationary point of C-2 symmetry was found to be 1.2 kJ mol(-1) below the energy of the C-s structure.