FROM MOLECULAR CLUSTERS TO BULK MATTER - I - STRUCTURE AND THERMODYNAMICS OF SMALL CO2, N-2, AND SF6 CLUSTERS

The thermodynamics and structural properties of small molecular X-N cl usters (X=N-2, 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. E vidence is provided for ''dynamical coexistence'' between solidlike an d liquidlike forms of the cluster, in a finite energy range, in the ca se of (CO2)(13) and (N-2)(13) but not (SF6)(13). In addition (N-2)(13) exibits a solid-solid phase transition characterized by the release o f the molecular orientational degree of freedom. A systematic use of t he dynamic quenching method enables us to interpret these different be haviors 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-worke rs, enables us to understand why different molecular clusters exhibit different crossover points from icosahedral to bulk properties. (C) 19 98 American Institute of Physics. [S0021-9606(98)00425-5].