Quasi-static deformation simulations of molecular crystals

Abstract

Identification of the mechanical performance of pharmaceuticals in the drug discovery process can determine the tabletability of a target molecule. Determination of the active slip systems and their ranking in molecular crystals is challenging because molecules offer a set of configurational variables absent from metallic or simple ionic materials, such as bond rotations, molecular rotations, and the relative orientation of molecules. This paper uses two computational methods, the rigid-block and tensor-based shearing methods, to calculate the slip barriers and gain insights regarding the slip deformation of simple molecular crystalline materials, using diatomic solid oxygen and anthracene as examples. Both methods use constrained quasi-static energy minimisation to simulate the materials' displacement and homogeneous shearing. These shearing methods rank the slip systems in oxygen and anthracene in agreement with experiment, including those reported herein where two previously unknown active slip systems in the basal plane of anthracene were identified independently from the computations. Internal degrees of freedom, in the form of shear-induced molecular rotations, critically influence the slip barriers and deformation mechanism. Our results uncover rotational twinning, which is linked to crystallographic symmetry rather than partial dislocations, while homogeneous shear of anthracene leads to a series of polymorphic transitions. The results also provide alternative interpretations of slip-observed morphologies.