空间解析热力学积分：计算稠密流体化学势的有效方法
Spatially Resolved Thermodynamic Integration: An Efficient Method to Compute Chemical Potentials of Dense Fluids
摘要：
计算化学势的许多流行方法都依赖于将测试粒子插入目标系统。在液体和液体混合物的情况下，随着密度或浓度的增加，这一程序的难度增加，因此不可避免地要使用复杂的强化取样技术。在这项工作中，我们提出了一种替代策略，空间分辨热力学积分，或简称SPARTIAN。在这里，分子在模拟子区域中以原子级分辨率描述，并且在较大的储层中作为理想气体颗粒。所有的分子都可以在子域之间自由扩散，动态地调整它们的分辨率。为了在整个模拟箱中实施均匀的密度分布，计算、施加单分子外部势，并通过两种分辨率之间的化学势差来识别。由于储层被表示为一个理想气体池，这个差值正好等于目标系统的过剩化学势。本方法超越了粒子插入方法的高密度/浓度限制，因为进入目标系统区域的理想气体分子自发地适应局部环境。理想气体表示对模拟的计算成本的贡献可以忽略不计，从而允许以最小的费用利用大的储层。该方法已通过计算过量化学势纯Lennard-Jones液体和混合物，SPC和SPC/E液体水，和氯化钠水溶液进行了验证。报告的结果很好地再现了这些系统的文献数据。
英文摘要：
Many popular methods for the calculation of chemical potentials rely on the insertion of test particles into the target system. In the case of liquids and liquid mixtures, this procedure increases in difficulty upon increasing density or concentration, and the use of sophisticated enhanced sampling techniques becomes inevitable. In this work we propose an alternative strategy, spatially resolved thermodynamic integration, or SPARTIAN for short. Here, molecules are described with atomistic resolution in a simulation subregion, and as ideal gas particles in a larger reservoir. All molecules are free to diffuse between subdomains adapting their resolution on the fly. To enforce a uniform density profile across the simulation box, a single-molecule external potential is computed, applied, and identified with the difference in chemical potential between the two resolutions. Since the reservoir is represented as an ideal gas bath, this difference exactly amounts to the excess chemical potential of the target system. The present approach surpasses the high density/concentration limitation of particle insertion methods because the ideal gas molecules entering the target system region spontaneously adapt to the local environment. The ideal gas representation contributes negligibly to the computational cost of the simulation, thus allowing one to make use of large reservoirs at minimal expenses. The method has been validated by computing excess chemical potentials for pure Lennard-Jones liquids and mixtures, SPC and SPC/E liquid water, and aqueous solutions of sodium chloride. The reported results well reproduce literature data for these systems.
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