Structure and Phase Transition in Systems of Large Molecules

在具有足够短吸引势的体系中，升华线超过汽液临界温度，液相不再以热力学稳定状态存在。对于具有更短距离势的体系，也会发生等结构的固-固转变。本文利用热力学微扰理论和蒙特卡罗模拟方法对这些反常相行为进行了系统的研究。主要的焦点放在富勒烯C_(60)上，发现这种物质只有在极低的温度范围内才有稳定的液相。然而，所涉及的温度很高(~2000K)，有待实验证实。第二个对象是C&lt；60&gt；固体，它是一种典型的塑性固体，表现出取向有序-无序转变，这是C&lt；60&gt；分子小各向异性的表现。我们提出了一个简单的模型，它为压力下的状态方程和取向跃迁提供了统一的图景。该模型由C_(60)和C_(60)固体的第一原理电子结构计算得到的两相的有效分子间势组成。本研究的第三个主题是关于用于相变的计算机模拟方法的局限性，发现在计算机模拟中由可视分子构型确定的相界与由共存热力学条件得到的相界有很大的不同。我们特别注意了C&lt；60&GT；的高温相图，发现以前基于可视化方法的一些模拟是错误的。

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.