TTCF4LAMMPS: A toolkit for simulation of the non-equilibrium behaviour of molecular fluids at experimentally accessible shear rates

We present TTCF4LAMMPS, a toolkit for performing non-equilibrium molecular dynamics (NEMD) simulations to study the fluid behaviour at low shear rates using the LAMMPS software. By combining direct NEMD simulations and the transient-time correlation function (TTCF) technique, we study the fluid response to shear rates spanning 15 orders of magnitude. We present two examples for simple monatomic systems: one consisting of a bulk liquid and another with a liquid layer confined between two solid walls. The small bulk system is suitable for testing on personal computers, while the larger confined system requires high-performance computing (HPC) resources. We demonstrate that the TTCF formalism can successfully detect the system response for arbitrarily weak external fields. We provide a brief mathematical explanation for this feature. Although we showcase the method for simple monatomic systems, TTCF can be readily extended to study more complex molecular fluids. Moreover, in addition to shear flows, the method can be extended to investigate elongational or mixed flows as well as thermal or electric fields. The high computational cost needed for the method is offset by the two following benefits: i) the cost is independent of the magnitude of the external field, and ii) the simulations can be made highly efficient on HPC architectures by exploiting the parallel design of the algorithm. We expect the toolkit to be useful for computational researchers striving to study the nonequilibrium behaviour of fluids under experimentally-accessible conditions.

我们提出了TTCF4LAMMPS——一款基于LAMMPS软件开展非平衡分子动力学（non-equilibrium molecular dynamics, NEMD）模拟的工具包，用于研究低剪切速率下的流体行为。该工具包结合直接非平衡分子动力学模拟与瞬态时间关联函数（transient-time correlation function, TTCF）技术，可针对跨越15个数量级的剪切速率范围分析流体响应。我们针对两类简单单原子系统提供了示例：其一为本体液体体系，其二为受限在两固壁之间的液体层体系。小型本体体系适合在个人计算机上开展测试，而更大规模的受限体系则需要高性能计算（high-performance computing, HPC）资源支持。我们证明了瞬态时间关联函数形式体系可成功检测任意弱外场下的体系响应，并针对这一特性给出了简要的数学解释。尽管我们仅以简单单原子系统展示了该方法的应用，但TTCF4LAMMPS可轻松拓展至更复杂的分子流体研究。此外，除剪切流外，该方法还可拓展用于研究拉伸流、混合流以及热场或电场场景。该方法较高的计算成本可通过以下两项优势得以抵消：i) 计算成本与外场强度无关；ii) 通过利用算法的并行设计，可在高性能计算架构上实现极高的模拟效率。我们期望该工具包能够为致力于在实验可及条件下研究流体非平衡行为的计算研究人员提供助力。