Departamento de Física Aplicada, Universidad de Vigo, 32004 Ourense, Spain.
J Chem Phys. 2010 Mar 7;132(9):094505. doi: 10.1063/1.3328667.
In this work, the melting point and the phase diagram of methanol is determined via computer simulations using the OPLS model. The three different solid structures that are found experimentally were considered. By computing the free energies of both the fluid phase and the three different solid structures (alpha, beta, gamma), the initial solid-solid and fluid-solid coexistence points were determined. By performing Gibbs-Duhem integration, the complete coexistence lines were evaluated. In this way, it was possible to compute, for the first time, the complete phase diagram for a potential model of methanol. It is found that the optimized potential model for liquid simulations (OPLS) provides reasonable predictions for the densities of the three solid polymorphs, although they tend to be somewhat low when compared with the experiment. Overall the model provides a qualitatively correct description of the phase diagram of methanol. The beta solid, which is thermodynamically stable in the experimental phase diagram of methanol, is found to be metastable in the phase diagram of the model. The alpha phase is stable at low pressures and the gamma phase is stable at high pressures, in agreement with experiment. Thus, the model is able to predict the existence of the gamma solid at high pressure. From free energy calculations we found that the melting point of the model at room pressure is 215 K. That was further confirmed by direct coexistence simulations. Thus, the model presents a melting point about 40 K above the experimental value of 175 K. Thus the OPLS model provides a reasonable description of the phase diagram of methanol, but it could probably be modified to improve the phase diagram predictions.
在这项工作中，通过使用 OPLS 模型的计算机模拟确定了甲醇的熔点和相图。考虑了实验中发现的三种不同的固体结构。通过计算流体相和三种不同固体结构（α、β、γ）的自由能，确定了初始固-固和液-固共存点。通过进行 Gibbs-Duhem 积分，评估了完整的共存线。通过这种方式，首次有可能计算甲醇的潜在模型的完整相图。结果表明，用于液体模拟的优化势能模型 (OPLS) 对三种固体多晶型物的密度提供了合理的预测，尽管与实验相比，它们往往略低。总体而言，该模型对甲醇的相图提供了定性正确的描述。在甲醇实验相图中热力学稳定的β固体，在模型相图中被发现是亚稳的。在低压下，α 相稳定，在高压下，γ 相稳定，与实验相符。因此，该模型能够预测高压下γ固体的存在。从自由能计算中，我们发现模型在室温下的熔点为 215 K。这进一步通过直接共存模拟得到了证实。因此，模型的熔点比实验值 175 K 高出约 40 K。因此，OPLS 模型对甲醇的相图提供了合理的描述，但可能需要进行修改以改善相图预测。
