Isomerization dynamics and ergodicity in Ar-7

The isomerization dynamics of a model Ar-7 cluster, bound by the pairwise Lennard-Jones potential, are examined in the melting and liquid-like regions We use molecular dynamics with frequent quenching to calculate the accumulated residence times and distribution of lifetimes in each of the four geometrical isomers as a function of the microcanonical energy. The residence times are compared with the ratios of the density of states associated with the isomers, evaluated by microcanonical Monte Carlo simulations, and are in good agreement. The distribution of lifetimes is found to be exponential (Poissonian) except at very short times. We propose and test a simple explanation for this behavior in terms of rapid recrossings of the dividing surface. The energy dependence of the individual isomerization rates is compared with the predictions of RRK theory through a knowledge of the transition states. We discuss the results in the light of a recent study that addressed the issue of ergodicity in this system. (C) 1997 American Institute of Physics. [S0021-9606(97)02144-2].