title: Comparison of perturbative and variational treatments of molecular vibrations: application to the vibrational spectrum of HFCO up to 8000 cm(-1).
abstract: We calculated highly excited states of the HFCO molecule, comparing results from two methods. In the first method, Van Vleck perturbation theory is used to transform away all off-diagonal couplings except those between nearly degenerate states. This perturbative transformation leads to a matrix representation where eigenvalues are obtained with relatively small matrices. In the second method, variational eigenvalues are obtained by combining the Jacobi-Wilson approach with the block-Davidson scheme. The key ingredient here is a prediagonalized-perturbative scheme applied to a subspace of a curvilinear normal-mode basis set. Comparisons of the two methods provide a critical test of the less time-consuming perturbation theory. Two different coordinate sets are used to test the sensitivity of the results to coordinate choice. Perturbation theory also requires a polynomial fit to the potential. The implications of this restriction are investigated.
authors.affiliation: Laboratoire de Structure et Dynamique des Systèmes Moléculaires et Solides (LSDSMS) Unité Mixte de Recherche (UMR-CNRS 5636)-CC 014, Université Montpellier II, 34095 Montpellier, Cedex 05, France. iung@univ-montp2.fr
authors.forename: Christophe
authors.initials: C
authors.lastname: Iung
authors.forename: Fabienne
authors.initials: F
authors.lastname: Ribeiro
authors.forename: Edwin L
authors.initials: EL
authors.lastname: Sibert