Identifying Possible Slip Systems of Molecular Crystals via a Geometry-Based Procedure

Crystal plasticity is achieved primarily through slipping. Here, a geometry-based procedure is developed to facilitate the identification of slip systems in arbitrarily oriented three-dimensional periodic molecular crystals. The procedure requires a molecular crystal structure as the only input, and a steric hindrance parameter denoted as the maximum atom overlap factor (MAOF) is used to quantify the steric hindrance experienced along a specific slip direction. Slip direction with the lowest MAOF is considered to be the most favorable. Under the constraint of crystal translational symmetry preservation, the searching space of the slip directions with low Miller indexes can be readily covered by efficient loop searches. Since the favorable slip planes of a molecular crystal can be readily predicted by the geometric analysis tool developed in the Cambridge Structural Database Python API, we can easily obtain the favorable slip systems following basic geometry. The procedure is validated by a structurally diverse set of molecular crystals through comparison with experimental and theoretical calculation results. While the procedure is highly efficient, it is intended for coarse filtering of slip systems due to its simplicity. In cases where high accuracy is desired, this procedure should be used in conjunction with other methods and further validated against accurate first-principles shear-slipping calculations. In addition, the crystal-level steric hindrance parameter MAOF in combination with the molecular-level parameter dissociation energy of the weakest bond is shown to be well correlated with the impact sensitivity of a variety of energetic molecular crystals.