raaSAFT: A framework enabling coarse-grained molecular dynamics simulations based on the SAFT-γ Mie force field

We describe here raaSAFT, a Python code that enables the setup and running of coarse-grained molecular dynamics simulations in a systematic and efficient manner. The code is built on top of the popular HOOMD-blue code, and as such harnesses the computational power of GPUs. The methodology makes use of the SAFT-γ Mie force field, so the resulting coarse grained pair potentials are both closely linked to and consistent with the macroscopic thermodynamic properties of the simulated fluid. In raaSAFT both homonuclear and heteronuclear models are implemented for a wide range of compounds spanning from linear alkanes, to more complicated fluids such as water and alcohols, all the way up to nonionic surfactants and models of asphaltenes and resins. Adding new compounds as well as new features is made straightforward by the modularity of the code. To demonstrate the ease-of-use of raaSAFT, we give a detailed walkthrough of how to simulate liquid–liquid equilibrium of a hydrocarbon with water. We describe in detail how both homonuclear and heteronuclear compounds are implemented. To demonstrate the performance and versatility of raaSAFT, we simulate a large polymer-solvent mixture with 300 polystyrene molecules dissolved in 42 700 molecules of heptane, reproducing the experimentally observed temperature-dependent solubility of polystyrene. For this case we obtain a speedup of more than three orders of magnitude as compared to atomistically-detailed simulations.本文介绍raaSAFT——一款可系统化、高效完成粗粒度分子动力学（coarse-grained molecular dynamics）模拟搭建与运行的Python代码。该代码基于主流的HOOMD-blue框架开发，可充分利用GPU的计算算力。其核心方法采用SAFT-γ Mie力场（SAFT-γ Mie force field），所生成的粗粒度成对势能与模拟流体的宏观热力学性质紧密关联且保持一致。raaSAFT支持涵盖直链烷烃、水与醇类等复杂流体，乃至非离子表面活性剂、沥青质与树脂模型在内的多种化合物的同核（homonuclear）与异核（heteronuclear）模型构建。该代码采用模块化设计，可便捷地添加新化合物与新功能。为展示raaSAFT的易用性，本文详细演示了烃类与水的液液平衡（liquid–liquid equilibrium）体系的模拟流程。本文还详细阐述了同核与异核化合物模型的具体实现方式。为验证raaSAFT的性能与通用性，本文模拟了一个大型聚合物-溶剂混合体系：将300个聚苯乙烯（polystyrene）分子溶解于42700个正庚烷（heptane）分子中，复现了实验观测到的聚苯乙烯温度依赖溶解度特性。相较于全原子精细模拟（atomistically-detailed simulations），该体系下raaSAFT的加速比可达三个数量级以上。