Ab initio potential energy surface for the low-frequency out-of-plane bending motions of the water trimer

A potential energy surface is presented for the out-of-plane bending angles χ1, χ2 and χ3 of the three non-hydrogen-bonded H atoms in the water trimer. Starting from a C3h planar reference geometry interaction energies are evaluated using a previously developed SCF + MP2/ESPB approach at 69 geometries spanning the χ1, χ2, χ3 space. These are fit to a sixth-degree 50-term polynomial expansion. The standard deviation of the fit is about 5 μ hartree. Stationary points are located and the barriers between them are determined. The physical origins of the barriers and the role of non-additivity are discussed on the basis of an energy partitioning of the interaction energy.