Local and normal modes: A classical perspective

Abstract

Normal and local mode behavior in molecular systems and the transition between them is explored using nonlinear mechanics techniques. Significant insight into this behavior and into the structure of phase space results from a generalized definition of local and normal modes and the associated identification of normal modes as a (1:1) resonance between local zeroth order oscillators. In addition to qualitative insight, this approach yields a simple formula [Eq. (28)] for the level of excitation at which local modes become possible. Applications to H₂O and to the overtone spectroscopy of the dihalomethanes, benzene, and TMS are provided.