Variational calculations of rotational–vibrational energy levels of water

Calculations are carried out of the rotational–vibrational energy levels of H2O and D2O for J≤10 by a variational method. The full Watson Hamiltonian is employed, with the potential function in valence displacement coordinates and with the integrations over normal coordinates carried out by Gauss–Hermite quadrature. The basis set consists of products of vibrational functions and symmetric top functions; the vibrational functions diagonalize the Hamiltonian for J=0. Comparison is made with experiment, and also results obtained with different force fields are compared. The mixing of different vibrational functions into a wave function for a given rotational–vibrational state is studied; mixing is found to be quite prevalent.