Using swapping layers in molecular dynamics simulations to drive structural equilibration far below T_{g}.

Abstract

The anomalous surface mobility exhibited by many slowly vapor-deposited molecular solids, and their simulated counterparts, allows annealing their structure and increasing their density down to T_{dep}≈0.85T_{g}. This work describes the design and characteristics of a simulation that uses a "swept swapping" technique designed to extend structural annealing to lower temperatures. Films are grown using size-dispersed attractive Lennard-Jones (L-J) potential spheres deposited onto their free surface and then equilibrated with a surface-following Monte Carlo swap region. The motion of that region combined with sphere size dispersion speeds structural relaxation by orders of magnitude compared to bulk swapping. Sphere size span and size step (25% and ∼2.5%, respectively) are sufficient to completely suppress crystallization and maintain a high swap acceptance rate. The regime revealed by this technique attains a high packing fraction, which is near that of an hexagonal-close-packed L-J crystal with no size-dispersion. Additional swapping refines its structure, at near constant density, maximizing short-range icosahedral order to levels higher than seen previously; the levels apparently limited only by the computing budget.