A VARIATIONAL METHOD FOR THE CALCULATION OF VIBRATIONAL-ENERGY LEVELS OF TRIATOMIC-MOLECULES USING A HAMILTONIAN IN HYPERSPHERICAL COORDINATES

A full variational procedure is presented which is particularly suited to the calculation of vibrational (J = 0) energy levels of triatomic potentials with large amplitude motions and also allows for high permutational symmetries. Starting from simple basis functions reflecting the structure of the kinetic energy operator, an optimized basis set is derived by a step-wise contraction scheme. The method is applied to H3+ and Na3+ (D3h) and to model potentials for H2O+ (C2-nu) and HLiH- (D-infinity-h). The results for H3+ are shown to be superior to all previous calculations, in particular for those energy levels that lie above 20 000 cm-1. A new ab initio potential is presented for Na3+, for which converged energy levels are calculated up to 3500 cm-1. For H2O+ and HLiH-, the calculated energy levels agree with those obtained from a variational procedure in internal valence coordinates.