GENERAL-METHOD FOR REMOVING RESONANCE SINGULARITIES IN QUANTUM-MECHANICAL PERTURBATION-THEORY

This paper presents a way of improving second-order perturbation theor y calculations by summing contributions of uncoupled excitations to in finite order. For problems involving molecular vibrations, the new the ory is shown to give similar results to conventional second-order pert urbation theory when the system treated has no near resonances but als o to give accurate and stable results even very close to resonance. Th e new theory is tested by comparison to converged variational calculat ions for vibrational energy levels of formaldehyde, formaldehyde-d(2), and two two-dimensional model subsystems based on formaldehyde. (C) 1 996 American Institute of Physics.