Molecular dynamics simulations of the order-disorder phase transition in adamantane

Molecular dynamics simulations are performed on a periodic arrangement of 256 adamantane molecules interacting via a pairwise additive atom-atom potential. The low temperature phase is reproduced successfully. The transition from a tetragonal to a cubic unit cell is found between 380 K and 420 K with a first-order loss of long-range orientational order observed between 410 K and 420 K. A study of correlation between the orientations of neighbouring molecules shows that short-range orientational order persists dynamically above the phase transition, suggesting that in a real system the plastic phase is never truly disordered. On cooling,the reverse transitionfrom plastic to ordered tetragonal phase is found to occur at 340 K.