Towards an ab initio flexible potential for water, and post-harmonic quantum vibrational analysis of water clusters

We describe a full-dimensional, flexible potential energy surface for arbitrary numbers of water monomers built from ab initio 2- and 3-body potentials. These potentials are each permutationally invariant fits to roughly 30 000 electronic energies. Tests of these potentials are made against direct high-level ab initio results for the water dimer and trimer. An application to the hexamer (prism isomer) is made along with a review of recent ab initio calculations of harmonic frequencies. One-mode quantum calculations of the OH-stretch fundamentals are done with these potentials and their accuracy is assessed by comparisons with coupled MULTIMODE calculations for the water dimer and trimer. Local-mode calculations of OH-stretch fundamentals are presented for the hexamer and decamer, where it appears that the bulk limit is being approached. A new local-monomer model is presented and tested for the water dimer and trimer.