MOLECULAR-DYNAMICS OF THE TRANSITION IN SF6

Very slow warming and cooling sequences are made in the molecular dynamics (MD) of the true crystal to plastic crystal phase transition in SF6. A parallel computer with 1024 processors (the AMT DAP) is used, each processor taking charge of three molecules. The Parrinello-Rahman (PR) zero-stress algorithm is used, starting with a perfect single crystal. The MD system is warmed and cooled in sequence twice between 30 K and 190 K, indicating a transition with a hysteresis. On warming, the plastic phase appears at 129 K +/- 6 K, but is better defined as 83 K +/- 3 K on cooling. In both the warming and cooling sequences it can be seen that the structure change proceeds through a rhombohedral structure which is metastable in the real system under certain conditions. In the non-equilibrium MD (NEMD) the metastable phase cannot be stabilised when using a large system and the PR algorithm, but its appearance is reproducible. Although the simulations are NEMD, entropy estimates are made using the velocity scaling procedure as a heat bath, and the discrepancies introduced because of the algorithm and the quasi-static approximation are seen to be remarkably small. The latent heat at the transition is about 10 kcal/mol. The extended frozen defect which appears in one cooling sequence has a very significant effect on the entropy function, showing that future studies of the entropy of defects through MD are a viable possibility.