A local mode potential function for the water molecule

Abstract

The molecular rotation-vibration hamiltonian is written in local coordinates and a variational technique for calculating accurate eigenfunctions and eigenvalues is described. It is shown that the local mode approach is rapidly convergent and no more difficult to apply than the normal mode method. A simple four-parameter Morse potential function is fitted to 56 vibrational levels of H₂O, HDO and D₂O, and used to predict all ro-vibrational levels of up to 5 vibrational quanta with J = 0, 1 and 2. The agreement between calculated and observed frequencies is remarkably good for a model containing so few empirical parameters.