Liquid-like Regime Research Articles

Diffusion in classical periodic systems: The Smoluchowski equation approach

The classical diffusion of a particle in a periodic system is studied employing the Smoluchowski equation with an external periodic field of force. Assuming a two-dimensional rectangular symmetry and an external potential of a simple cosine shape, the resulting eigenvalue problem leads to a Hill equation, which is solved numerically. The dynamic structure factor and its full width at half maximum (fwhm) are calculated up to large values of the momentum transfer extending in several Brillouin zones. It is shown that, decreasing the amplitude of the potential, the Smoluchowski equation describes a diffusive process which changes continuously from a jump to a liquid-like regime, with an intermediate behaviour which has some characteristics of both. In this context, recent scattering experiments on systems (premelting surfaces and absorbed monolayers) which seem to show such a behaviour, are briefly discussed.

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.