Molecular dynamics of the transition in SF6

Abstract

Abstract 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 10kcal/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.