Dynamical instabilities and structural changes in molecules

Structural instabilities in molecular clusters have been observed in several recent classical simulations. They arise from changes in the dynamical interactions among the vibrational modes as energy increases and are entirely unrelated to vibrationally-induced instabilities of Jahn-Teller type. We show that the self-consistent field (SCF) approximation for vibrations provides a simple predictive model for such instabilities, in which the onset of instability is shown to arise from changes in the effective (mean) potential acting on each mode, as the total energy changes. Exact classical dynamics was used to test the SCF prediction for a simple two-mode model, with the result that the instability onset is correctly predicted by the SCF approximation. We comment on the relevance of these mean potentials for characterizing related phase transitions in clusters.