Collective Variables for Conformational Polymorphism in Molecular Crystals

Controlling polymorphism in molecular crystals is crucial in the pharmaceut-ical, dye, and pesticide industries. However, its theoretical description is extremely challenging, due to the associated long time scales (>1 mu s). 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 (similar to 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.

Automated summary

Controlling polymorphism in molecular crystals is challenging due to long time scales. We propose an efficient method for identifying collective variables that promote transitions between conformational polymorphs. Using this method in simulations, we observe transitions between all solid polymorphs of CL-20 and identify previously unknown intermediate forms.