Elements: Enabling Accurate Thermal Transport Calculations in LAMMPS

计算热输运研究对于开发能够解决具有挑战性的能源和环境问题的新材料至关重要。分子动力学(MD)模拟被广泛用于研究材料中的热输运。大规模原子/分子大规模并行模拟器(LAMMPS)是目前应用最广泛的分子动力学软件包之一。本项目的主要目标是在LAMMPS中创建并仔细实施改进的热传输计算方法。该软件拥有数十万用户，解决方案必须合并到LAMMPS核心代码中，而不是以模块化的“插件”形式提供，这一事实使这一问题变得具有挑战性。该项目的三个目标是：(1)对LAMMPS中所有支持的多体势能实施修正的热流计算，(2)确定受改变的热流计算影响最大的分子系统类型，以及(3)教育LAMMPS社区如何正确地实现新势能中的热流，以及培训新的科学家为LAMMPS代码库贡献专业质量的代码。除了其他广泛的影响外，这项研究将使大规模的材料计算筛选能够准确地预测其热性能，这实现了材料基因组倡议(MGI)的关键目标之一。此外，它是一个创新的、可扩展的、可重复使用的软件组件，通过对本科生的培训支持广泛的LAMMPS用户社区的培训和一般劳动力的发展，并确保通过开放源码的LAMMPS包广泛提供新的软件功能。此外，该项目通过一名训练有素的常驻软件开发人员和匹兹堡大学研究计算中心的资源为研究生和本科生提供专业的软件工程培训。由于LAMMPS的庞大用户基础和开源性质，这项研究预计将产生广泛的影响；这些软件创新将在工业和学术界广泛使用。计算热导率的最常见的MD技术，Green-Kubo方法，在LAMMPS中对大多数分子模拟产生不正确的结果。尽管使用LAMMPS发表的数千篇论文的热通量误差的后果尚未完全确定，但初步数据表明，对于液态碳氢化合物，通过多体势能的热通量可能被低估95%，导致热通量的总误差为22%。除非这个广泛使用和高度优化的软件包能够实现和实施正确的热输运计算，否则具有新热学性质的材料的研究和开发将受到严重阻碍。目前还没有广泛可用的能够正确计算热通量的MD程序。一个例外是只存在成对相互作用的分子系统(这排除了所有具有两个以上原子的分子)，在这种情况下，目前的LAMMPS实现给出了正确的结果。LAMMPS中的热流计算问题早在四年多前就被发现了，但在任何广泛使用的MD软件包中缺乏正确的实现说明了寻找和指定软件工程师来完成这项重要工作的挑战。该项目阐述了在LAMMPS中为多体势能扩展重新实施热通量计算的理论、实施路径和验证策略。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为是值得支持的。

Computational thermal transport research is critical to the development of new materials that can address challenging energy and environmental problems. Molecular dynamics (MD) simulations are used extensively to study thermal transport in materials. One of the most widely used MD software packages is the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). It is the primary aim of this project to create and carefully implement improved thermal transport calculation methods in LAMMPS. This problem is made challenging by the fact that this software has hundreds of thousands of users and the solution must be merged into the core LAMMPS code, as opposed to offered as a modular "plug-in". The three objectives of the project are: (1) to implement a corrected heat flux computation for all supported many-body potentials in LAMMPS, (2) to identify the types of molecular systems most affected by the changed heat flux computations, and (3) educate the LAMMPS community on how to implement heat flux in new potentials correctly as well as train new scientists to contribute professional-quality code to the LAMMPS code base. This research will, among other broad impacts, enable large-scale computational screening of materials to accurately predict their thermal properties, which fulfills one of the key goals of the Materials Genome Initiative (MGI). Furthermore, it is an innovative, scalable, reusable software component that supports training for the broad LAMMPS user community as well as general workforce development via training to undergraduates, and ensures the new software capacities are widely available via the open-source LAMMPS package. Additionally, the project provides professional software engineering training to graduate and undergraduate students via a highly-trained resident software developer and resources from the Center for Research Computing at the University of Pittsburgh. Due to the large user base and open source nature of LAMMPS, the research is expected to have a broad impact; these software innovations will be widely available across both industry and academia.The most common MD technique to compute thermal conductivity, the Green-Kubo method, yields incorrect results in LAMMPS for the majority of molecular simulations. Although the ramifications of the error in the heat flux for the thousands of papers already published using LAMMPS has yet to be fully determined, preliminary data indicates that for liquid hydrocarbons, the heat flux through a many-body potential can be underreported by 95%, leading to a total error of the heat flux of 22%. Unless correct thermal transport calculations can be achieved and implemented for this widely used and highly optimized software package, research and development of materials with novel thermal properties will be significantly hindered. There is no widely available MD code that is able to correctly compute heat flux. The one exception is for molecular systems where only pair-wise interactions exist (which excludes all molecules with greater than two atoms), in which case the current LAMMPS implementation gives correct results. The heat flux computation problem in LAMMPS was identified more than four years ago, but the lack of a correct implementation in any widely used MD software package speaks to the challenge of finding and dedicating software engineers to do this important work. This project addresses the theory, implementation pathway, and validation strategy for an expansive re-implementation of heat flux computations in LAMMPS for many-body potentials.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.