Morrow Timothy I, Maginn Edward J
Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, USA.
J Chem Phys. 2005 Feb 1;122(5):54504. doi: 10.1063/1.1839172.
An extended system molecular dynamics method for the isomolar semigrand ensemble (fixed number of particles, pressure, temperature, and fugacity fraction) is developed and applied to the calculation of liquid-liquid equilibria (LLE) for two Lennard-Jones mixtures. The method utilizes an extended system variable to dynamically control the fugacity fraction xi of the mixture by gradually transforming the identity of particles in the system. Two approaches are used to compute coexistence points. The first approach uses multiple-histogram reweighting techniques to determine the coexistence xi and compositions of each phase at temperatures near the upper critical solution temperature. The second approach, useful for cases in which there is no critical solution temperature, is based on principles of small system thermodynamics. In this case a coexistence point is found by running N-P-T-xi simulations at a common temperature and pressure and varying the fugacity fraction to map out the difference in chemical potential between the two species A and B (mu(A)-mu(B)) as a function of composition. Once this curve is known the equal-distance/equal-area criterion is used to determine the coexistence point. Both approaches give results that are comparable to those of previous Monte Carlo (MC) simulations. By formulating this approach in a molecular dynamics framework, it should be easier to compute the LLE of complex molecules whose intramolecular degrees of freedom are often difficult to properly sample with MC techniques.
开发了一种用于等摩尔半巨正则系综（固定粒子数、压力、温度和逸度分数）的扩展系统分子动力学方法，并将其应用于两种 Lennard-Jones 混合物的液-液平衡（LLE）计算。该方法利用一个扩展系统变量，通过逐渐改变系统中粒子的身份来动态控制混合物的逸度分数 xi。使用两种方法来计算共存点。第一种方法使用多直方图重加权技术来确定在上临界溶液温度附近温度下各相的共存 xi 和组成。第二种方法适用于不存在临界溶液温度的情况，基于小系统热力学原理。在这种情况下，通过在共同的温度和压力下运行 N-P-T-xi 模拟并改变逸度分数，以绘制出两种物种 A 和 B 之间化学势的差异（μ(A)-μ(B)）作为组成的函数，从而找到共存点。一旦知道这条曲线，就使用等距离/等面积准则来确定共存点。两种方法得到的结果都与先前的蒙特卡罗（MC）模拟结果相当。通过在分子动力学框架中制定这种方法，应该更容易计算复杂分子的 LLE，其分子内自由度通常很难用 MC 技术进行适当采样。
