Instantaneous normal mode analysis as a probe of cluster dynamics

We report an analysis of dynamical transitions in small argon clusters based on a study of the vibrational frequencies (photon spectra) of these systems. Even in the liquidlike regime such an analysis can be shown to provide an exact description of the short-time cluster dynamics and represents an alternative to more conventional strategies which concentrate on an enumeration of minimum energy structures. The overall picture of ‘‘melting’’ transitions emerging from this study is one of a series of isomerizations which preserve the short-range structures of the clusters, with the structures linked by these isomerizations sometimes being far from any of the local minima on the potential energy hypersurface. As a part of the analysis, we describe a general method for estimating cluster atom self-diffusion constants from system configurations obtained via either isothermal or isoergic Monte Carlo calculations.