Structural transitions in small molecular clusters

Clusters of octahedral molecules mimicking TeF6 afford a vehicle to investi gate the analog in small clusters of structural phase transitions through m olecular dynamics simulations. Three phaselike forms occur for the solid cl usters, both closed-shell structures of 51, 89, and 137 molecules, and open -shell structures of 50, 81, and 129 molecules. The indications are that th e free energy has at least two minima as a function of the order parameter. However, whether these converge to a single minimum as N, the number of mo lecules in the cluster, grows large, leading to a second-order transition i n the bulk, or remain apart, implying at least a weak first-order transitio n in the bulk limit, cannot be determined from the simulations. (C) 1999 Am erican Institute of Physics. [S0021-9606(99)51507-9].