Calculation of Vibrational Wavefunctions and Energies Using MRD-CI Techniques

Abstract

The calculation of electronic wavefunctions is an important objective in quantum chemistry but it is evident that if even this could be accomplished without any approximation it would still not be sufficient to explain many types of molecular structure phenomena observed experimentally. In the spectra of molecular systems, for example, the intensity associated with a given electronic transition is often spread over a wide range of wavelength and in this instance the most that one can hope to obtain from a calculation of the electronic energy for the participating states at a single geometry is the approximate location of an absorption (or emission) maximum found therein. To go beyond the simple calculation of vertical spectra it is clearly necessary to introduce at least vibrational motion into the theoretical treatment and this objective is most easily accomplished using the Born-Oppenheimer Method [1], or, as it is more commonly referred to among spectroscopists, the Franck-Condon Approximation [1].