Molecular dynamics simulation of the phase transition in adamantane

Abstract

Phase changes in adamantane have been studied by MD simulation on the DAP computers, using a zero-pressure technique to simulate clusters of 128 and 256 molecules where each member interacts with all others via the rigid molecule model and the 6-exp atom–atom potential. The form of the potential has been modified to permit the use of the 16 hydrogen sites only, giving a 65% saving in the calculation times. This model is shown to give lattice dynamics of adamantane closely similar to results with potentials which are generally accepted.Using this potential the system equilibrates into the correct low temperature phase [Formula: see text] and on heating, a transition is observed at 210 ± 10 K to an Fm3m phase where the molecules lie preferentially in the Td orientations, as expected. Further heating beyond 240 ± 15 K removes all apparent orientational order, though the underlying lattice is still fcc. On recooling the cluster from 300 to 100 K the orientational distribution function developed a significant degree of order as determined through the calculation of a correlation function designed to show any local order. This order is consistent with the lowest phase structure, but would in itself be insufficient to suggest a particular crystal structure.