Research on twin boundaries (TBs) in molecular crystals has primarily emphasized their effect on mechanical properties and applications, with limited focus on microstructural details. Twin identification typically relies on extensive experiments, lacking reliable computational predictions. Here, we extend to molecular crystals the previously developed evolutionary methodology for predicting grain boundary structures in atomic crystals. To achieve that, care must be taken of molecular orientations, conformations, and grain displacements during global optimization. We apply this method to predict possible TBs on the (100), (010), and (001) planes of polymorph I of aspirin; the (100), (010), (001), (102), (110), and (210) planes of α-RDX; and the (010), ($10\overline{1}$), (011), (101), and (110) planes of β-HMX. For HMX, the lowest-energy (101) TB matches experimentally reported twin, validating our approach. We predict unexpected low-energy TB configurations, including a unique cluster-like structure on (100) plane of aspirin and numerous conformational changes in RDX. Finally, we discuss the characteristics and patterns of molecular crystal twin formation.

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• Crystal structure
• Interfaces
• Molecular solids
• Materials modeling
• Molecular dynamics