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

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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.