Rough Mob – Roughness and Mobility of Coarse-Grained Molecular Models

软物质系统的分子模拟越来越需要多尺度方法，以便既准确又计算可行。粗粒度模型可以通过几种系统的方式从原子参考模拟中生成。它们很好地再现了母原子模拟的静态结构分布。然而，它们共同未能捕获分子的流动性和随后的传输量，如扩散系数，粘度或某些特征时间。它们的流动性往往高出几个数量级。我们提出了一种方法（“粗糙暴民”）来预测这种人工加速的分子流动性，作为一个系统的原子模型被替换为一个粗粒度的模型。它的前提是，通过将一个化学部分的几个真实的原子结合成一个超原子，它的一些表面结构会丢失：一个粗糙的原子团被平滑成一个无结构的球形超原子。我们进一步断言，正是这种表面粗糙度的降低，使得光滑的超原子比粗糙的原子部分更快地滑过彼此。这个想法已经在我们自己发表的一组脂肪族和芳香族碳氢化合物溶剂的初步工作中得到了验证，这些溶剂都被粗粒化成一个单一的超原子。结果表明，有一个定义良好的线性关系之间的一个适当定义的纯几何粗糙度原子模型和粗粒度模型和所观察到的流动性增加之间的差异。这是非常令人鼓舞的，因为它开辟了一种预测输运系数的方法，通过在粗粒度模拟中计算输运系数，然后使用Rough Mob加速因子将其重新缩放为现实值，而不需要昂贵或不可行的原子计算。在拟议的工作中，这种方法将被推广到一个更大的系统和条件：（i）系统，在粗粒度的水平，需要多个超原子，包括聚合物。(ii)不同化学物质的混合物。(iii)不同的状态点（温度、密度、成分）。将对所有这些情况进行验证。最后，粗糙暴徒的概念是比较其他两个尝试预测先验的加速度粗粒化：过剩熵缩放和近似Mori-Zwanzig方法。该提案的最终目的是有助于寻求分子模拟方法，这些方法能够可靠地计算复杂软物质系统的流变学和输运性质。它超越了纯粹的学术兴趣：有了这些方法，聚合物挤出和注塑成型等工艺可以设计得更具成本效益和能源效益。

The molecular simulation of soft-matter systems increasingly needs multiscale methods in order to be both accurate and computationally feasible. Coarse-grained models can be generated in several systematic ways from atomistic reference simulations. They reproduce static structural distributions of the parent atomistic simulations very well. They collectively fail, however, to capture the molecular mobility and the ensuing transport quantities like the diffusion coefficient, the viscosity or certain characteristic times. Their mobility is often several orders of magnitude too high. We propose a method (“Rough Mob”) to predict this artificial acceleration of the molecular mobility, as the atomistic model of a system is replaced by a coarse-grained model. Its premise is that, by uniting several real atoms of a chemical moiety into one superatom, some of its surface structure is lost: A rough atomistic group of atoms is smoothened into a structure-less spherical superatom. We further assert that it is this reduction in surface roughness, which enables smooth superatoms to glide past one another faster than rough atomistic moieties. This idea has been tested in our own published preliminary work on a set of aliphatic and aromatic hydrocarbon solvents, which have all been coarse-grained into a single superatom. It was shown that there is a well-defined linear relation between a suitably defined purely geometric roughness difference between atomistic and coarse-grained models and the observed mobility increase. This is very encouraging, as it opens a way of predicting transport coefficients by calculating them in a coarse-grained simulation and then using the Rough Mob acceleration factor to rescale them to realistic values without the need for costly or infeasible atomistic calculations. In the proposed work, this approach is to be generalised to a larger set of systems and conditions: (i) Systems which, at the coarse-grained level, need multiple superatoms, including polymers. (ii) Mixtures of different chemical species. (iii) Different state points (temperature, density, composition). Validations are to be carried out for all these cases. Finally, the Rough Mob concept is to be compared to two other attempts to predict a priori the acceleration upon coarse-graining: excess-entropy scaling and approximate Mori-Zwanzig approaches. The ultimate aim of this proposal is to contribute to the quest for molecular simulation methods, which are able to calculate reliably the rheological and transport properties of complex soft-matter systems. It goes beyond purely academic interest: With such methods available, processes such as polymer extrusion and injection moulding could be designed to be both more cost-effective and energy-effective.