METHODS FOR CALCULATION OF ELECTRONIC VIBRATIONAL-SPECTRA OF POLYATOMIC-MOLECULES .3. MODEL-CALCULATIONS AND COMPARISON WITH OTHER TECHNIQUES

Two new methods for computing the vibrational structure of electronic spectra of complex polyatomic molecules have been suggested previously in Parts I and II. The peculiarities of their practical implementatio n are considered in the present paper. The results of a large number o f model calculations are also discussed. The types and magnitudes of p ossible errors are analyzed for both the methods and ways of enhancing the accuracy, and error control is suggested. It is shown that despit e their approximate character, the accuracy of these methods is of the same order as that of the so-called ''exact'' methods used before (1 cm(-1) for vibrational frequencies and 5% for relative intensities), a nd the speed of operation is more than two orders higher than that of the previous methods. This fact essentially enhances the possibility o f reliable and plausible interpretation of experimental spectra and op ens the way for the statement and the solution of common inverse spect ral problems in the theory of polyatomic molecules' vibronic spectra.