Controlling polymorphism in molecular crystals is crucial in the pharmaceutical, dye, and pesticide industries. However, its theoretical description is extremely challenging, due to the associated long time scales (>1 μs). We present an efficient procedure for identifying collective variables that promote transitions between conformational polymorphs in molecular dynamics simulations. It involves applying a simple dimensionality reduction algorithm to data from short (∼ps) simulations of the isolated conformers that correspond to each polymorph. We demonstrate the utility of our method in the challenging case of the important energetic material, CL-20, which has three anhydrous conformational polymorphs at ambient pressure. Using these collective variables in Metadynamics simulations, we observe transitions between all solid polymorphs in the biased trajectories. We reconstruct the free energy surface and identify previously unknown defect and intermediate forms in the transition from one known polymorph to another. Our method provides insights into complex conformational polymorphic transitions of flexible molecular crystals.

Nitroaromatic 2,4-dinitroanisole (DNAN) was used as a model compound for the study of surface-poisoning effects in crystallization. In addition to the expected solvent effect, it was found that the concentration of solutions and presence of additives control the formation of polymorphs and forms. In some cases, aromatic and/or nitro-containing additives interact with DNAN, probably competing with intrinsic intermolecular interactions that allow the formation and growth of DNAN crystals. It was also found that singlecrystal-to-single-crystal phase transitions take place between two β forms and from the β form to the α form. This β-to-α transformation is most probably possible because of a similarity between the two crystal structures. Hence, the barrier for this transformation is rather low. Nevertheless, the formation of α-DNAN in the typically β morphology introduces strain in the crystals, causing defects. It was also found that the orientations of the molecules against the unit cell faces for the two α forms (blocks and needles) are markedly different.

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