Elements: Enabling Accurate Thermal Transport Calculations in LAMMPS

计算热输运研究对于解决具有挑战性的能源和环境问题的新材料的开发至关重要。分子动力学（MD）模拟被广泛用于研究材料中的热传递。最广泛使用的MD软件包之一是大规模原子/分子大规模并行模拟器（LAMMPS）。本项目的主要目的是在LAMMPS中创建并仔细实施改进的热输运计算方法。由于该软件有成千上万的用户，并且解决方案必须合并到LAMMPS核心代码中，而不是作为模块化“插件”提供，因此这个问题变得具有挑战性。该项目的三个目标是：(1)实现LAMMPS中所有支持的多体势的修正热通量计算，(2)确定受改变的热通量计算影响最大的分子系统类型，以及(3)教育LAMMPS社区如何正确实现新势的热通量，并培训新的科学家为LAMMPS代码库贡献专业质量的代码。除了其他广泛的影响外，这项研究将使材料的大规模计算筛选能够准确预测其热性能，这实现了材料基因组计划（MGI）的关键目标之一。此外，它是一个创新的、可扩展的、可重用的软件组件，通过对本科生的培训，支持广泛的LAMMPS用户社区的培训以及一般劳动力的发展，并确保新的软件能力通过开源的LAMMPS包广泛可用。此外，该项目通过一个训练有素的常驻软件开发人员和来自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的庞大用户群和开源性质，该研究预计将产生广泛的影响；这些软件创新将广泛应用于工业界和学术界。在大多数分子模拟中，最常见的计算热导率的MD技术，Green-Kubo方法，在LAMMPS中产生不正确的结果。虽然使用LAMMPS已经发表的数千篇论文的热流密度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，通过多体势的热流密度可能被低估95%，导致热流密度的总误差为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传递计算，否则具有新型热性能的材料的研究和开发将受到严重阻碍。没有广泛可用的MD代码，能够正确地计算热通量。唯一的例外是只存在成对相互作用的分子系统（排除了所有大于两个原子的分子），在这种情况下，当前的LAMMPS实现给出了正确的结果。LAMMPS中的热流密度计算问题早在四年前就被发现了，但是在任何广泛使用的MD软件包中都缺乏正确的实现，这说明了寻找和奉献软件工程师来完成这项重要工作的挑战。本项目解决了理论、实现途径和验证策略，以便在LAMMPS中广泛地重新实现多体势的热通量计算。该奖项反映了美国国家科学基金会的法定使命，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。

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.