Structure and Phase Transition in Systems of Large Molecules

在具有足够短程吸引势的系统中，升华线在液-汽临界温度以上通过，液相不再以热力学稳定状态存在。对于具有更进一步的短程势的体系，也会发生等构固-固转变。本文采用热力学微扰理论和Monte Carlo模拟方法对这些反常的相行为进行了系统的研究。重点研究了富勒烯C_2<60>，发现它只有在很低的温度范围内才有稳定的液相。然而，所涉及的温度很高（约2000 K），有待实验证实。第二个课题是C_<60>solid，它是一种典型的塑性固体，表现出取向有序-无序的转变，这是C_solid的小的各向异性的表现<60>。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转变提供了一个统一的图象。该模型是利用已有的C_solid第一原理电子结构计算方法构造的两相有效分子间势模型<60>.本文的第三个问题是计算机模拟方法的局限性，发现计算机模拟中由可视分子构型确定的相界与用计算机模拟得到的相界有很大的不同.本文的第四个问题是，用计算机模拟的方法得到的相界与用计算机模拟的方法得到的相界有很大的差别，而用计算机模拟的方法得到的相界与用计算机模拟的方法得到的相界有很大的差别共存的热力学条件我们特别关注了C_2的高温相图<60>，发现以前的一些基于可视化方法的模拟是错误的。

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.