STRUCTURE OF WATER CLUSTERS - THE CONTRIBUTION OF MANY-BODY FORCES, MONOMER RELAXATION, AND VIBRATIONAL ZERO-POINT ENERGY

The vibrationally averaged properties of small water clusters from the dimer to the hexamer are discussed, The potential energy surface used contains explicit many-body terms which allows the non-pairwise inter actions to be considered. The ground vibrational states are calculated accurately using a diffusion quantum Monte Carlo algorithm which give s vibrationally averaged rotational constants in good agreement with e xperiment, The many-body forces cause a destabilization of the more do sed structures, and there is a significant variation in the intermolec ular zero-point energies for different structures. Cyclic structures a re easily the most stable for the trimer and tetramer; in the latter c ase, this is probably due to the three- and four-body forces, The cycl ic pentamer is also probably the structure with the highest dissociati on energy when all effects are considered. For the hexamer, a noncycli c cagelike structure is found to be most stable and its stability is d ue to a relatively low zero-point energy.