Polymorph Specific Order Parameters: An Analysis of Surface Melted Layer Thickness for Glycine Nuclei

We developed order parameters that distinguish between polymorphs in molecular crystals and their nuclei. The order parameter values for each molecule are based on the root mean squared deviation (RMSD) between its local environment and polymorph specific templates. We demonstrate that the RMSD order parameters can distinguish the α, β, and γ-glycine polymorph crystal structures in the bulk and in solvated nuclei. The structures of solvated glycine nuclei are examined using the newly developed order parameters. We show that the solvated α-glycine nucleus has a thinner surface melted layer than the γ-glycine nucleus. We suggest that using simulations to compute the surface melted layer thickness may provide insights on polymorph selection.