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 interactions to be considered. The ground vibrational states are calculated accurately using a diffusion quantum Monte Carlo algorithm which gives vibrationally averaged rotational constants in good agreement with experiment, The many-body forces cause a destabilization of the more dosed structures, and there is a significant variation in the intermolecular zero-point energies for different structures. Cyclic structures are easily the most stable for the trimer and tetramer; in the latter case, this is probably due to the three- and four-body forces, The cyclic pentamer is also probably the structure with the highest dissociation energy when all effects are considered. For the hexamer, a noncyclic cagelike structure is found to be most stable and its stability is due to a relatively low zero-point energy.