Uncoupled effective Hamiltonians for molecules with several vibrational modes coupled by Coriolis and centrifugal terms

In this paper we extend methods we have developed [J. Chem. Phys. 94 (1991) 461; 98 (1993) 83] for studying the spectra of molecules with two modes coupled by Coriolis and centrifugal terms. Combining a transformation method due to Bogoliubov and Tyablikov (BT) and perturbation theory we calculate ro-vibrational energy levels of molecules (formal-dehyde and thioformaldehyde) with three coupled modes and a molecule (trans-d2 ethylene) with several coupled pairs of modes. The BT transformation is used to compute "vibrational" basis functions which include some of the rotational-vibrational coupling. © 1997 Published by Elsevier Science B.V.