CHAOTIC DYNAMICS AND VIBRATIONAL-MODE COUPLING IN SMALL ARGON CLUSTERS - THE INFLUENCE OF POTENTIAL-ENERGY SADDLES

We have computed the local Kolmogorov entropy of molecular dynamics tr ajectory segments near the potential energy saddles of model Ar3 and A r5 clusters. In the case of Ar3 clusters bound with a Lennard-Jones po tential, the local Kolmogorov entropy of the cluster is significantly smaller in the saddle region than in other areas of the potential surf ace. This behavior indicates an increase in the degree of nearly quasi periodic motion near the Ar3 saddle due to the partial decoupling of t he cluster's vibrational modes there. Lennard-Jones Ar5 clusters do no t 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 surfac e near the saddle. From these observations, we can determine which fea tures of the saddle are most important in this respect; the flatness o f the saddle region seems to be one such feature.