3-BODY EFFECTS ON MOLECULAR-PROPERTIES IN THE WATER TRIMER

We report an application of diffusion Monte Carlo to investigate the i mportance of three-body forces on the properties of the water trimer. The potential energy surface used is due to Millot and Stone and is ba sed on intermolecular perturbation theory to which three-body inductio n and dispersion energies are added. The effects of the three-body for ces are considered by comparison with the same potential containing on ly pairwise water interactions. We have calculated minimum energy stru ctures, vibrationally averaged structures, zero-point energies, rotati onal constants, cluster dissociation energies, and tunneling splitting s, with and without the three-body forces. The values obtained for the vibrationally averaged rotational constants with the three-body poten tial are fairly close to the experimental values. Whereas the rotation al constants are shown to have a significant dependence, the tunneling splittings are changed little by the three-body forces. Based on the calculated difference in anharmonic zero-point energies in water dimer and trimer, we predict that vibrational excitation of a stretching mo de will cause predissociation in (H2O)(3) but probably not in (D2O)(3) . (C) 1995 American Institute of Physics.