Here we present a comprehensive evaluation of a set of wellknown all-atom force fields with the scope to model dynamic phenomena in molecular crystals composed of polyaromatic hydrocarbons. The capability of the force fields to reproduce experimentally and computationally available data is thoroughly scrutinized against anthracene molecular crystals that serve as a model system. First, the properties of the solid crystalline phase are investigated by employing geometry optimization using molecular mechanics. Because any inaccuracies can be easily overlooked in the constrained solid crystalline phase, the interaction energy of a variety of dimer conformations is obtained by employing an extensive local minima search algorithm. The larger configurational freedom in the dimer conformations better reflects the incorporation of molecules at the surface during crystal growth. The results are compared to known ab initio calculations as very little experimental data concerning the anthracene dimer are available. Finally, for three force fields with different performance in other tests, a polymorph prediction is carried out. Overall, we show that some of the selected all-atom force fields (BMM2, BMM3, W99, and isoPAHAP) perform remarkably well, whereas others (Amber, Bordat, Dreiding, DRESP, MM2, and MM3) fail to reproduce known computational data for a variety of reasons.
The trajectories of a single nitrogen molecule resulting from a series of collisions with coronene molecular clusters of varying size are determined numerically by means of classical molecular dynamics simulations at two system temperatures, corresponding to the clusters being in solid and liquid state. The observed bimodality of the residence time distributions that corresponds to a combination of specular and diffuse molecular scattering tends to disappear with increasing temperature due to the more rapid rearrangements of the coronene cluster constituent molecules in the liquid state. The mean residence time decreases with increasing system temperature and appears to be independent of the coronene cluster size within the cluster size-range considered here. The recorded trajectories of the nitrogen probe are relatively tortuous, on average one order of magnitude longer than the shortest path connecting the impact and desorption points. The vast majority of the sites visited during the nitrogen molecule residence period correspond to the atoms at the edge of coronene molecules, mainly hydrogens. The intermolecular cohesive forces between the molecules cause that the coronene clusters are impenetrable by the nitrogen probe at temperatures below their thermal dissociation point. Highlights: • Collisions between nitrogen molecule and coronene clusters are investigated via classical molecular dynamics simulation. • Residence time distributions of the nitrogen probe exhibit bimodality, corresponding to a combination of specular and diffuse molecular scattering regimes. • Nitrogen trajectories are found to be highly tortuous with the majority of atomic sites visited belonging to the edge of the coronene molecules. • Coronene clusters are shown to be impenetrable below their thermal dissociation point.
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