Chaotic dynamics and vibrational mode coupling in small argon clusters

We have computed the local Kolmogorov entropy of molecular dynamics trajectory segments near the potential energy saddles of model Ar₃ and Ar₅ clusters. In the case of Ar₃ clusters bound with a Lennard-Jones potential, the local Kolmogorov entropy of the cluster is significantly smaller in the saddle region than in other areas of the potential surface. This behavior indicates an increase in the degree of nearly quasiperiodic motion near the Ar₃ saddle due to the partial decoupling of the cluster's vibrational modes there. Lennard-Jones Ar₅ clusters do not exhibit similar behavior, but Ar₅ clusters bound with a short-range Morse potential do. This suggests that the “regularizing” effect of saddle regions is strongly dependent on the shape of the energy surface near the saddle. From these observations, we can determine which features of the saddle are most important in this respect; the flatness of the saddle region seems to be one such feature.