ζ-Glycine: Insight into the Mechanism of a Polymorphic Phase Transition

Glycine is the simplest amino acid and a great test-bed for studying polymorphism in molecular crystals. With five phases having been structurally characterized at atmospheric or high pressure, it is a well-studied system both experimentally and theoretically. Yet a sixth form, the ζ phase, was discovered over a decade ago as a short-lived intermediate upon decompression of a high-pressure phase. However, its structure has remained unsolved despite several theoretical investigations to glycine polymorphism. We have recently reported [1] the structure of the elusive ζ phase, which was resolved thanks to the collaboration between crystal structure prediction procedure based on fully ab initio total energy calculations combined with a genetic algorithm, and neutron powder diffraction data obtained on a sample trapped at 100 K. Here we will discuss the approach that led to the successful theoretical prediction of this phase, the lessons that can be learnt and applied to similar systems, and mention remaining challenges for a predictive ab initio crystal structure prediction process.
[1] C. L. Bull et al. IUCrJ 4, 569 (2017)