Towards an accurate intermolecular potential for water

A new ab initio potential for the water dimer is presented. It is obtained from monomer properties and perturbation theory calculations (IMPT) on the dimer. The long range electrostatic interaction is described by distributed multipoles: charge, dipole and quadrupole on oxygen and charge and dipole on hydrogen. The induction is modelled using polarizabilities on oxygen up to quadrupole. The short range repulsive energy is modelled by an exponential site-site function incorporating site anisotropy. Two models have been used for the dispersion energy: accurate dispersion coefficients for the water dimer with Tang-Toennies damping functions, and a site-site model. Neither is entirely satisfactory. The geometry of the global minimum on the potential surface is in very good agreement with the experimental result. The virial coefficient agrees moderately well with the experimental determination; the discrepancies suggest that the potential is not deep enough.