An ab Initio Study of Observed and Hypothetical Polymorphs of Glycine

Abstract

To investigate the merits of crystal structure prediction using ab initio computational techniques, we have used density functional (DFT) methods to investigate the relative stabilities of the four known crystalline phases of glycine and also a range of alternative putative crystal structures of the zwitterion. Energy differences are calculated using a range of exchange-correlation functionals, and it is found that the calculated relative stability of the phases is sensitive to the choice of functional. Energy differences are found to be on the order of a few tenths of a kilocalorie per mole with little separation in energy found between observed and putative structures. This result is similar to that typically obtained from force field calculations and confirms the difficulty of the task of predicting the structure of molecular crystals. Optimization of structures, including optimization of unit cells, highlights the limitation of DFT in describing the long-range dispersion interaction. Use of the local density approximation (LDA) is shown to over-bind crystals, whereas use of gradient-corrected functionals severely underbinds crystals. Calculated structural energy differences are presented, which show that, for the case of the LDA, the four observed glycine polymorphs receive a lower energy than all putative glycine structures considered.

Many‐Body Dispersion Interactions in Molecular Crystal Polymorphism

“…As shown in Ref. 22, the local‐density approximation (LDA)23 underestimates the unit cell volumes by 7–10 %, while the generalized‐gradient approximation of Perdew, Burke, and Ernzerhof (PBE)24, 25 overestimates the unit cell volumes by 7–8 % 22. Adding the pairwise TS energy to the PBE functional reduces the error in the unit cell volumes to about 3 %, a significant improvement.…”

“…The LDA and PBE data were taken from Ref. 22. Note that LDA underestimates the unit cell volumes, while PBE overestimates them.…”

“… Potential‐energy surfaces for the a ‐ b plane of γ‐Gly,31 obtained using: a) PBE, b) PBE+TS, and c) PBE+MBD. Experimental lattice parameters are marked by a cross 22. The experimental error bars are not visible on this scale ( BE =binding energy).…”

The high-pressure electronic structure of the [Ni(ptdt)2] organic molecular conductor

“…Our results demonstrate that the PW91 GGA XC functional is capable of providing a sufficiently accurate description of the molecular structure, in agreement with a number of studies. 25,26,29 We turn now to discuss the effect of pressure upon the crystal and molecular structure. Examining the high pressure lattice parameters in Table I, it is apparent that the largest compression observed is along the c-axis, which experiences an approximate 22% reduction in magnitude, compared to 15 only provide structural data at ambient pressure (albeit at two distinct temperatures); consequently, there are no experimental data with which to compare our high-pressure structures.…”

Polymorphism and Solvatomorphism 2005

“…Density functional methods have been used to evaluate the use of ab initio computational techniques for crystal structure prediction, using the four polymorphs of glycine as the system for study 16. It was found that the calculated relative stability of the phases was sensitive to the choice of function, and only small energy differences were found between observed and predicted structures.…”