Molecular dynamics simulations on a parallel computer plastic crystals and related systems

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Four molecular dynamics projects concerning dynamic molecular disorder are discussed, all having in common the fact that they are being done on a computer which is an array of parallel processors. The computer architecture is ideal for these systems as the molecules do not undergo translational diffusion.The first system is SF6 both in the plastic phase and below, where two phase transitions have been identified. In the plastic phase orientational disorder is apparent, the molecules arranging themselves so as to minimise the effect of the frustration between the nearest neighbour attraction and the next-nearest neighbour repulsion. The phase transitions represent the compromises enforced by this frustration as the temperature is lowered. A time correlation analysis shows that it is unlikely that phonon-like modes can exist in the plastic phase.The second system is naphthalene very near to melting, where the predominant molecular reorientation is about the axis of greatest inertia. This is contrasted with experimental results which suggest that the reorientation about the axis of least inertia is associated with melting, the other reorientations not being catastrophic for the crystallinity of the system. Arguments are presented that the system does not have a plastic phase just below the melting point.The third system is butane, where the molecule itself has internal degrees of freedom which are incorporated in the model. A number of crystal phases have been discovered, most of which must be metastable. On warming the stable triclinic phase to a temperature above the plastic transition in the real system, the model system takes on an orthorhombic structure where the symmetry is a time average.The last system discussed is an invented two-dimensional system of molecules we call quaternane, as they have four atoms arranged in a chain. Each molecule has two internal degrees of freedom both of which carry a double-well potential. The system develops domain walls when warmed from a low-temperature crystal phase, and it is apparent that the structure within the walls shows smectic liquid-crystal-like behaviour. This model could well prove to be a starting point for liquid-crystal simulations at the atomic level.