Validation of missed space-group symmetry in X-ray powder diffraction structures with dispersion-corrected density functional theory

Abstract

More than 600 molecular crystal structures with correct, incorrect and uncertain space-group symmetry were energy-minimized with dispersion-corrected density functional theory (DFT-D, PBE-D3). For the purpose of determining the correct space-group symmetry the required tolerance on the atomic coordinates of all non-H atoms is established to be 0.2 Å. For 98.5% of 200 molecular crystal structures published with missed symmetry, the correct space group is identified; there are no false positives. Very small, very symmetrical molecules can end up in artificially high space groups upon energy minimization, although this is easily detected through visual inspection. If the space group of a crystal structure determined from powder diffraction data is ambiguous, energy minimization with DFT-D provides a fast and reliable method to select the correct space group.

Ambiguous structure determination from powder data: four different structural models of 4,11-difluoroquinacridone with similar X-ray powder patterns, fit to the PDF, SSNMR and DFT-D

“…Nowadays, typical errors in single-crystal structure analyses include wrong space groups (see e.g. Baur & Kassner, 1992;Herbstein et al, 2002;Marsh & Clemente, 2007;Henling & Marsh, 2014;Hempler et al, 2017); overlooked, ignored or wrongly treated disorder; missing hydrogen atoms [e.g. Cp*CoCoCp* instead of Cp*CoH 3 CoCp* (Cp* = C 5 (CH 3 ) 5 ) (Kersten et al, 1992)]; misplaced hydrogen atoms; or wrong atom assignments, e.g.…”

Elucidating the Crystal Structure of dl-Arginine by Combined Powder X-ray Diffraction Data Analysis and Periodic DFT-D Calculations

“…Although the task of determining crystal structures from powder XRD data is significantly more challenging than from single-crystal XRD data, advances over the past 20 years or so in techniques for analysis of powder XRD data − (particularly, the development of the direct-space strategy for structure solution) are such that the structural properties of organic materials of moderate complexity can now be determined from powder XRD data. In this regard, we note that there is growing interest ,− in utilizing periodic DFT calculations in conjunction with the analysis of powder XRD data, both for validation of the structural model and for enhancing the quality of the structure at different stages of the structure determination process. The capability to determine the crystal structures of organic materials directly from powder XRD data has created considerable opportunities for structural characterization of materials prepared by processes that intrinsically generate microcrystalline powders as the product phase, including those prepared by solid-state desolvation (e.g., dehydration) processes. ,,,, …”