Collective Variables for Conformational Polymorphism in Molecular Crystals

Area of Science:

• Materials Science
• Computational Chemistry
• Crystallography

Background:

• Controlling molecular crystal polymorphism is vital for pharmaceuticals, dyes, and pesticides.
• Theoretical descriptions are challenging due to long simulation time scales (>1 μs).

Purpose of the Study:

• To develop an efficient procedure for identifying collective variables (CVs) that accelerate transitions between conformational polymorphs.
• To apply this method to understand the complex polymorphism of the energetic material CL-20.

Main Methods:

• Applying a dimensionality reduction algorithm to short (∼ps) simulations of isolated conformers.
• Utilizing identified CVs in Metadynamics simulations to observe polymorph transitions.
• Reconstructing the free energy surface to identify intermediate and defect forms.

Main Results:

• Successfully identified CVs that promote transitions between conformational polymorphs.
• Observed transitions between all solid polymorphs of CL-20 in biased simulations.
• Discovered previously unknown defect and intermediate forms during polymorph transitions.

Conclusions:

• The developed method efficiently identifies CVs for studying molecular crystal polymorphism.
• Provides significant insights into the complex conformational transitions of flexible molecular crystals.
• Applicable to various industries requiring control over crystal forms.