Structure and Phase Transition in Systems of Large Molecules

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In systems with sufficiently short-ranged attractive potentials the sublimation line passes above the liquid-vapor critical temperature and the liquid phase no more exists as a thermodynamically stable state. The isostructual solid-solid transition also occurs for systems with further short-ranged potentials. In the present study systematic investigation is made for these unusual phase behaviors using a thermodynamic perturbation theory and Monte Carlo simulation method. Main focus is put on the fulleren C_<60> and it is found that this substance has a stable liquid phase only in the very nallow range of temperature. However, the temperature concerned is very high (〜2000K) to be confirmed experimentally. The isostuctural solid-solid transition is expected to occur, at least in principle, in colloids.The second subject is C_<60> solid which is a typical plastic solid and shows the orientational order-disorder transition, which is a manifestation of small anisotropy of C_<60> molecules. We proposed a simple model which provides a unified picture for the equaution of state and orientational transition under pressure. This model consists of the effective intermolecular potentials constructed for both phases using the existing first-principle electronic structure calculations for C_<60> solid.The third subject in the present study is concerned with limitation of the computer simulation method used for the phase transition and it is found that the phase boundaries determined by the visual molecular configurations in computer simulations are quite different from those obtained by the thermodynamic condition of coexistence. We paid a special attention to the high-temperature phase diagram of C_<60> and found that some of the previous simulations based on the visual method are wrong.