Computational study of transition dynamics in 55-atom clusters

Abstract

Molecular dynamics computer simulation has ben employed to study structure and isomerization dynamics of intact 55-atom clusters. The interactions used were selected to represent the heavier noble gases Ar, Kr, and Xe. As an aid for interpretation of results, the molecular dynamics computation was coupled to steepest-descent mapping to locate relevant cluster inherent structures (potential energy minima). A relatively sharp melting transition has been reproducibly observed. In its low-temperature ‘‘solid state’’ the cluster predominately inhabits the basins for the Mackay icosahedral inherent structure, with occasional excursions into and out of particle–hole states (an atom promoted from filled second to empty third icosahedral shell). Most inherent structures for the liquid droplet state are amorphous, are higher in energy than those for the solid, have no obvious icosahedral ancestry, and display surface capillary excitations. Freezing can produce defective solid structures which then can be annealed to the ground-state icosahedral structure. Root-mean-square distances under mapping to minima have been evaluated vs temperature; they show behavior qualitatively similar to, but quantitatively shifted from, the bulk-phase behavior prescribed by the Lindemann melting criterion and its conjugate freezing criterion.