Sandia National Laboratories , Albuquerque, New Mexico 87185, United States.
U.S. Naval Research Laboratory , Washington, D.C. 20375, United States.
J Chem Theory Comput. 2017 Jun 13;13(6):2890-2896. doi: 10.1021/acs.jctc.7b00241. Epub 2017 Jun 2.
The distinctive viscoelastic behavior of polymers results from a coupled interplay of motion on multiple length and time scales. Capturing the broad time and length scales of polymer motion remains a challenge. Using polyethylene (PE) as a model macromolecule, we construct coarse-grained (CG) models of PE with three to six methyl groups per CG bead and probe two critical aspects of the technique: pressure corrections required after iterative Boltzmann inversion (IBI) to generate CG potentials that match the pressure of reference fully atomistic melt simulations and the transferability of CG potentials across temperatures. While IBI produces nonbonded pair potentials that give excellent agreement between the atomistic and CG pair correlation functions, the resulting pressure for the CG models is large compared with the pressure of the atomistic system. We find that correcting the potential to match the reference pressure leads to nonbonded interactions with much deeper minima and slightly smaller effective bead diameter. However, simulations with potentials generated by IBI and pressure-corrected IBI result in similar mean-square displacements (MSDs) and stress autocorrelation functions G(t) for PE melts. While the time rescaling factor required to match CG and atomistic models is the same for pressure- and non-pressure-corrected CG models, it strongly depends on temperature. Transferability was investigated by comparing the MSDs and stress autocorrelation functions for potentials developed at different temperatures.
聚合物独特的粘弹性行为是由多个长度和时间尺度上的运动耦合相互作用产生的。捕捉聚合物运动的广泛时间和长度尺度仍然是一个挑战。我们使用聚乙烯 (PE) 作为模型大分子，构建了具有每个 CG 珠三到六个甲基的 CG 模型，并探究了该技术的两个关键方面：迭代 Boltzmann 反演 (IBI) 后需要进行压力修正，以生成与完全原子熔体模拟压力匹配的 CG 势能，以及 CG 势能在温度范围内的可转移性。虽然 IBI 产生的非键对势能在原子和 CG 对相关函数之间给出了极好的一致性，但 CG 模型的压力与原子系统的压力相比仍然很大。我们发现，通过修正势能来匹配参考压力，会导致非键相互作用具有更深的势阱和稍小的有效珠径。然而，使用 IBI 和压力修正 IBI 生成的势能进行的模拟，对于 PE 熔体，均会导致相似的均方根位移 (MSD) 和应力自相关函数 G(t)。虽然匹配 CG 和原子模型所需的时间缩放因子对于压力和非压力修正的 CG 模型是相同的，但它强烈依赖于温度。通过比较在不同温度下开发的势能的 MSD 和应力自相关函数，研究了可转移性。
