Perturbation treatment of local-mode stretching vibrations

We show that perturbation theory by the method of moments is suitable for t he calculation of vibrational energy levels for models of local-mode stretc hing vibrations, characterized by strongly mixed, degenerate unperturbed st ates. Simple recurrence relations give the perturbation corrections to the energy and moments of the selected state. We explicitly consider the vibrat ional energy levels of three- and four-dimensional coupled Morse-oscillator models, parametrized for the stretching modes of ammonia and silane, respe ctively, and address the problem of avoided crossings. (C) 2001 Elsevier Sc ience B.V. All rights reserved.