SELF-CONSISTENT-FIELD PERTURBATION-THEORY OF MOLECULAR VIBRATIONS

Perturbation theory is used to perform non-iterative calculations of e nergy eigenvalues of the coupled ordinary differential equations that result from imposing separability assumptions in terms of normal coord inates on vibrational wavefunctions. Various model Hamiltonians with 2 or 3 coupled normal modes are studied and the increase of computation al cost with the number of degrees of freedom is analysed. Quadratic P ade approximants of the perturbation expansions are rapidly convergent , and directly yield complex numbers for resonance eigenvalues. For a 3-mode system, results are obtained within partial separability assump tions, with a pair of modes left coupled. Large-order perturbation the ory with partial separability is suggested as an alternative to low-or der exact perturbation theory.