Development of Molecular Simulation Methods to Compute Phase and Interfacial Properties of Complex Fluids

纽约州立大学布法罗分校的Jeffrey Errington和Andrew Schultz获得了化学系化学理论、模型和计算方法项目的奖励，以开发计算方法和工具来预测复杂分子系统的性质。 该项目由工程部化学，生物工程，环境和运输系统计划中的计算和数据支持科学与工程共同资助。 该项目侧重于相（气体，液体，固体）和界面（例如空气和水相遇）特性。 这些性质在许多自然现象和许多工业过程中起着关键作用。 对科学家和工程师特别有价值的是理解微观相互作用（例如原子水平）与其所表现出的宏观行为之间的关系。 这些信息可以用来调整系统的分子水平细节，以获得所需的行为。 从原理上讲，分子模拟为研究复杂流体的相态和界面行为提供了理想的工具。 虽然在这一领域已经取得了巨大的进步，仍然是一个巨大的需求，高效和有效的计算方法。 Errington和Schultz教授及其团队正在开发强大的新策略，通过分子模拟来询问复杂流体的相和界面性质。举例说明这种方法的必要性，包括分离技术的设计，能量存储设备，碳捕获策略和表面涂层。 这项研究的结果将在软件中实现，该软件可免费提供给更广泛的研究社区。本研究的重点是发展分子模拟方法，使人们能够推断复杂流体的体积和界面性质。两种方法的进展正在进行中：（1）一个新的严格的策略，用于计算流体的整体液体-蒸汽饱和特性和（2）基于力的策略，以确定在等温等压系综内的扩展界面势。 对于第一，维里压力测量收集在多个密度，跨越液体-蒸气共存区域，随后用于构建等温等压系综内的体积概率分布。该方法提供了与常用的平坦直方图方法相同的信息水平，不需要分子插入/缺失，并且可以在分子动力学框架内实现。 第二个方法上的进步是为了增加传播界面势方法的可及性。界面电位提供了关于系统润湿行为的定性和定量方面的重要见解。一般的方法提供了一种方法来确定在母蒸气中的固体基底上的液滴的接触角。该项目解决了在常用的等温等压系综内实施该方法所面临的挑战。标准方法导致高度拉长的模拟框，在实践中难以使用。研究小组已经确定了一种方法，可以通过使用虚拟盒子来显着减少所需的模拟盒子的大小。该方法利用基于力的策略来计算界面势。当与以前的发展相结合，这一进步提供了一个严格的，有效的，和访问的方法来确定模型系统的润湿性能。在第三个努力，开发工具，以促进耦合的Monte Carlo和分子动力学算法在一个单一的分子模拟框架。这种耦合增强了上述方法的效率。具体来说，该小组正在为可免费获得的LAMMPS分子动力学模拟器提供额外的Monte Carlo移动类型。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Jeffrey Errington and Andrew Schultz of SUNY at Buffalo are supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry to develop computational methods and tools to predict properties of complex molecular systems. This project is co-funded by the Computational and Data-Enabled Science and Engineering in Chemical, Bioengineering, Environmental and Transport Systems Program in the Division of Engineering. The project focuses on phase (gas, liquid, solid) and interfacial (e.g. where air and water meet) properties. The properties play a key role in many natural phenomena and in numerous industrial processes. Of particular value to scientists and engineers is an understanding of the relationship between the microscopic interactions, for example at the atomic level, and the macroscopic behavior it exhibits. Such information can be used to tune the molecular-level details of a system to obtain a desired behavior. In principle, molecular simulation provides an ideal tool for studying the phase and interfacial behaviors of complex fluids. Although tremendous advancements have been made in this area, there is still a huge need for efficient and effective computational methods. Professors Errington and Schultz and their groups are developing robust new strategies for interrogating the phase and interfacial properties of complex fluids via molecular simulation. Examples that illustrate the need for such methods include the design of separation technologies, energy storage devices, carbon capture strategies, and surface coatings. The results of this research will be implemented in software that is freely available to the broader research community. The focus of this research is to develop molecular simulation methods that enable one to deduce the bulk and interfacial properties of complex fluids. Two methodological advances are being pursued: (1) a new rigorous strategy for computing the bulk liquid-vapor saturation properties of fluids and (2) a force-based strategy to determine the spreading interface potential within an isothermal-isobaric ensemble. For the first, virial pressure measurements are collected at multiple densities that span the liquid-vapor coexistence region, and subsequently used to construct a volume probability distribution within the isothermal-isobaric ensemble. The method provides the same level of information as commonly-used flat histogram approaches, does not require molecule insertions/deletions, and can be implemented within a molecular dynamics framework. The second methodological advance is aimed at increasing the accessibility of the spreading interface potential method. The interface potential provides important insight regarding qualitative and quantitative aspects of a system's wetting behavior. The general approach provides a means to determine the contact angle of a liquid droplet on a solid substrate in a mother vapor. The project addresses challenges associated with implementing the method within the commonly-used isothermal-isobaric ensemble. The standard approach results in highly elongated simulation boxes that are difficult to work with in practice. The research team has identified a means to significantly reduce the size of the simulation box required via use of a virtual box. The approach leverages force-based strategies to compute the interface potential. When combined with previous developments, this advance provides a rigorous, efficient, and accessible approach for determining the wetting properties of model systems. In a third effort, tools are developed to facilitate coupling of Monte Carlo and molecular dynamics algorithms within a single molecular simulation framework. Such a coupling enhances the efficiency of the methods noted above. Specifically, the group is contributing additional Monte Carlo move types to the publically- and freely-available LAMMPS Molecular Dynamics Simulator. These contributions are expected to be beneficial to the broad LAMMPS user community.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.