Eike David M, Maginn Edward J
Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, USA.
J Chem Phys. 2006 Apr 28;124(16):164503. doi: 10.1063/1.2188400.
A method recently developed to rigorously determine solid-liquid equilibrium using a free-energy-based analysis has been extended to analyze multiatom molecular systems. This method is based on using a pseudosupercritical transformation path to reversibly transform between solid and liquid phases. Integration along this path yields the free energy difference at a single state point, which can then be used to determine the free energy difference as a function of temperature and therefore locate the coexistence temperature at a fixed pressure. The primary extension reported here is the introduction of an external potential field capable of inducing center of mass order along with secondary orientational order for molecules. The method is used to calculate the melting point of 1-H-1,2,4-triazole and benzene. Despite the fact that the triazole model gives accurate bulk densities for the liquid and crystal phases, it is found to do a poor job of reproducing the experimental crystal structure and heat of fusion. Consequently, it yields a melting point that is 100 K lower than the experimental value. On the other hand, the benzene model has been parametrized extensively to match a wide range of properties and yields a melting point that is only 20 K lower than the experimental value. Previous work in which a simple "direct heating" method was used actually found that the melting point of the benzene model was 50 K higher than the experimental value. This demonstrates the importance of using proper free energy methods to compute phase behavior. It also shows that the melting point is a very sensitive measure of force field quality that should be considered in parametrization efforts. The method described here provides a relatively simple approach for computing melting points of molecular systems.
一种最近开发的基于自由能分析来严格确定固液平衡的方法已被扩展用于分析多原子分子体系。该方法基于使用伪超临界转变路径在固相和液相之间进行可逆转变。沿着这条路径进行积分可得到单个状态点的自由能差，然后可用于确定自由能差随温度的变化关系，从而在固定压力下确定共存温度。本文报道的主要扩展是引入了一个外部势场，该势场能够诱导分子的质心有序以及二级取向有序。该方法用于计算1 - H - 1,2,4 - 三唑和苯的熔点。尽管三唑模型给出了液相和晶相准确的体密度，但发现它在再现实验晶体结构和熔化热方面表现不佳。因此，它得出的熔点比实验值低100 K。另一方面，苯模型已经经过广泛参数化以匹配多种性质，得出的熔点仅比实验值低20 K。之前使用简单“直接加热”方法的工作实际上发现苯模型的熔点比实验值高50 K。这证明了使用适当的自由能方法来计算相行为的重要性。它还表明熔点是力场质量的一个非常敏感的度量，在参数化工作中应予以考虑。这里描述的方法为计算分子体系的熔点提供了一种相对简单的途径。
