Institute for Computational Molecular Science, Temple University, Philadelphia, Pennsylvania 19122, United States.
Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States.
J Phys Chem B. 2023 Apr 27;127(16):3682-3689. doi: 10.1021/acs.jpcb.3c00805. Epub 2023 Apr 13.
A stringent test of the accuracy of empirical force fields is reproducing the phase diagram of bulk phases and mixtures. Exploring the phase diagram of mixtures requires the detection of phase boundaries and critical points. In contrast to most solid-liquid transitions, in which a global order parameter (average density) can be used to discriminate between two phases, some demixing transitions entail relatively subtle changes in the local environment of each molecule. In such cases, finite sampling errors and finite-size effects make the identification of trends in local order parameters extremely challenging. Here we analyze one such example, namely a methanol/hexane mixture, and compute several local and global structural properties. We simulate the system at various temperatures and study the structural changes associated with demixing. We show that despite a seemingly continuous transformation between mixed and demixed states, the topological properties of the H-bond network change abruptly as the system crosses the demixing line. In particular, by using spectral clustering, we show that the distribution of cluster sizes develops a fat tail (as expected from percolation theory) in the vicinity of the critical point. We illustrate a simple criterion to identify this behavior, which results from the emergence of large system-spanning clusters from a collection of aggregates. We further tested the spectral clustering analysis on a Lennard-Jones system as a standard example of a system with no H-bonds, and also, in this case, we were able to detect the demixing transition.
对经验力场准确性的严格检验是再现体相和混合物的相图。探索混合物的相图需要检测相界和临界点。与大多数固-液相变不同，在固-液相变中，可以使用全局序参量（平均密度）来区分两个相，一些分相转变涉及到每个分子的局部环境的相对细微的变化。在这种情况下，有限的抽样误差和有限尺寸效应使得识别局部序参量的趋势变得极具挑战性。在这里，我们分析了这样一个例子，即甲醇/己烷混合物，并计算了几个局部和全局结构性质。我们在不同的温度下模拟系统，并研究与分相相关的结构变化。我们表明，尽管在混合态和分相态之间似乎存在连续的转变，但氢键网络的拓扑性质在系统穿过分相线时会突然发生变化。具体来说，通过使用谱聚类，我们表明，在临界点附近，簇大小的分布会出现胖尾（如从渗流理论所预期的那样）。我们说明了一个简单的准则来识别这种行为，这种行为源于从聚集体中出现的大系统跨越簇。我们还在 Lennard-Jones 系统上测试了谱聚类分析，作为没有氢键的系统的标准示例，并且在这种情况下，我们也能够检测到分相转变。
