From molecular clusters to bulk matter. I. Structure and thermodynamics of small CO2, N2, and SF6 clusters

The thermodynamics and structural properties of small molecular XN clusters (X=N2, CO2, and SF6) are investigated using molecular dynamics simulations. In this paper we compare the behavior of carbon dioxide, nitrogen, and sulfur hexafluoride for a given cluster size of N=13. Evidence is provided for “dynamical coexistence” between solidlike and liquidlike forms of the cluster, in a finite energy range, in the case of (CO2)13 and (N2)13 but not (SF6)13. In addition (N2)13 exibits a solid–solid phase transition characterized by the release of the molecular orientational degree of freedom. A systematic use of the dynamic quenching method enables us to interpret these different behaviors in terms of the energy distribution of minima in the potential energy surface of the systems. A comparison of the strain energies of these clusters, using a model recently proposed by Wales and co-workers, enables us to understand why different molecular clusters exhibit different crossover points from icosahedral to bulk properties.