Isomorph invariance in the liquid and plastic-crystal phases of asymmetric-dumbbell models

TL;DR

This study numerically investigates asymmetric dumbbell models, demonstrating that their structure and dynamics are approximately invariant along isomorphs in both liquid and plastic-crystal phases, validating isomorph theory predictions.

Contribution

It provides the first validation of isomorph invariance in plastic crystals, showing better invariance than in the liquid phase for asymmetric-dumbbell models.

Findings

Structure and dynamics are isomorph invariant in reduced units.

Rotational autocorrelation varies more along isotherms than isomorphs.

Isomorph invariance applies better to plastic crystals than liquids.

Abstract

We present a numerical study of the asymmetric dumbbell model consisting of ``molecules'' constructed as two different-sized Lennard-Jones spheres connected by a rigid bond. In terms of the largest (A) particle radius, we report data for the structure and dynamics of the liquid phase for the bond lengths 0.05, 0.1, 0.2, and 0.5, and analogous data for the plastic-crystal phase for the bond lengths 0.05, 0.1, 0.2, and 0.3. Structure is probed by means of the AA, AB, and BB radial distribution functions. Dynamics is probed via the A and B particle mean-square displacement as functions of time and via the rotational time-autocorrelation function. Consistent with the systems' strong virial potential-energy correlations, the structure and dynamics are found to be isomorph invariant to a good approximation in reduced units, while they generally vary considerably along isotherms of the same (20%) density variation. Even the rotational time-autocorrelation function, which due to the constant bond length is not predicted to be isomorph invariant, varies more along isotherms than along isomorphs. Our findings provide the first validation of isomorph-theory predictions for plastic crystals for which isomorph invariance, in fact, is found to apply better than in the liquid phase of asymmetric-dumbbell models.