NOVEL METHODS FOR CALCULATING THE FINE VIBRONIC SPECTRA OF POLYATOMIC-MOLECULES

Novel approximate methods for calculating the vibrational structure of the electronic spectra of polyatomic molecules - a method for the dir ect calculation of the overlap integrals of vibrational wave functions for the electronic states involved in a transition and a variational method for the solution of the vibrational problem for the excited sta tes - are discussed The methods are based on the consideration of the displacement and entanglement of normal coordinates, the quasiorthogon ality of the Dushinsky transformation, and the classification of the s tates by total vibrational quantum numbers. Matrix perturbation theory is employed. It is shown that the accuracy of these methods compares well with the accuracy of the available ''exact'' techniques (the erro rs are similar to 1 cm(-1) for frequency and 10% for relative intensif y). At the same time, calculations by the new methods are performed mo re than two orders of magnitude faster than by the previously known me thods.