MOLECULAR-DYNAMICS SIMULATIONS OF THE ORDER-DISORDER PHASE-TRANSITIONIN 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 o rder observed between 410 K and 420 K. A study of correlation between the orientations of neighbouring molecules shows that short-range orie ntational order persists dynamically above the phase transition, sugge sting that in a real system the plastic phase is never truly disordere d. On cooling, the reverse transition from plastic to ordered tetragon al phase is found to occur at 340 K.