POTENTIAL-ENERGY SURFACE OF H...H2O

We investigated the potential energy surface for the H2O...H interaction in the van der Waals well region. Calculations were carried out using the Moller-Plesset second- and fourth-order perturbation theory in a [12s,7p,2d] --> (6s,5p,2d) basis set for the O atom, and [6s,2p,1d] --> (5s,2p,1d) for the H atoms. Basis set and superposition error effects were analyzed to gauge the reliability of the calculated potential. The potential was investigated in five physically distinct directions. The deepest potential well was found in the H2O molecular plane 3.30-3.45 angstrom from the H2O center of mass, near the H end of the OH bond. The following parameters are suggested for the spherically averaged potential: well depth 53 +/- 6 cm-1; minimum distance from the center of mass 3.25-3.40 angstrom.