Nuclear Vibrations and Force Constants

While valuable information on chemical bonding can be obtained from studies of the static structure of a bound system, even more understanding can be obtained from the internal restoring forces which control molecular vibrational motions and the mechanical response of molecules to external forces. This chapter deals with the description of such motions in terms of molecular harmonic and anharmonic vibrational force constants. Methods are described for the experimental evaluation of these parameters from spectroscopy, but the primary emphasis is on their computational determination since this approach can give more complete and accurate information for medium-sized polyatomic molecules. Consideration is given to techniques which can provide the highest possible accuracy within the harmonic oscillator approximation for polyatomic molecules and also the progress that has been made in fully anharmonic evaluation of the molecular vibrational potential energy hypersurfaces and the anharmonic energy levels on these surfaces.