Abstract
This paper describes the validation of a dispersion-corrected density functional theory (d-DFT) method for the purpose of assessing the correctness of experimental organic crystal structures and enhancing the information content of purely experimental data. 241 experimental organic crystal structures from the August 2008 issue of Acta Cryst. Section E were energy-minimized in full, including unit-cell parameters. The differences between the experimental and the minimized crystal structures were subjected to statistical analysis. The r.m.s. Cartesian displacement excluding H atoms upon energy minimization with flexible unit-cell parameters is selected as a pertinent indicator of the correctness of a crystal structure. All 241 experimental crystal structures are reproduced very well: the average r.m.s. Cartesian displacement for the 241 crystal structures, including 16 disordered structures, is only 0.095 Å (0.084 Å for the 225 ordered structures). R.m.s. Cartesian displacements above 0.25 A either indicate incorrect experimental crystal structures or reveal interesting structural features such as exceptionally large temperature effects, incorrectly modelled disorder or symmetry breaking H atoms. After validation, the method is applied to nine examples that are known to be ambiguous or subtly incorrect.
Highlights
In principle, theoretical calculations could supply independent data about molecular crystal structures to complement experimental data
Vexpected is the expected unit-cell volume, TExp is the temperature at which the crystal structure was measured, Vd-density functional theory (DFT) is the unit-cell volume after energy-minimization, Td-DFT is the apparent temperature of the dispersion-corrected density functional theory (d-DFT) method and k is a linear expansion coefficient; k and Td-DFT are the parameters that were fitted
Expressed as averages instead of r.m.s.d.s, the average discrepancy in the unit-cell volume with the d-DFT method was À1.1%, in good agreement with the À1.0% from the original d-DFT paper;2 the reader is reminded that the dispersion-correction parameters were parameterized against low-temperature crystal structures, and a small contraction of experimental unit cells upon energy minimization is expected
Summary
Theoretical calculations could supply independent data about molecular crystal structures to complement experimental data. This idea is certainly not new and there are ample examples in the literature (i) As a supplement to low-quality or medium-quality experimental data such as powder diffraction data, especially when preferred orientation is present. This is relevant for crystal structures for which high-quality experimental data cannot be obtained, as may be the case for metastable polymorphs, for crystals measured in a diamond–anvil cell, for crystal structures of highly insoluble compounds such as organic pigments or for co-crystals obtained through grinding. B66, 544–558 research papers (iv) To decide between two structural models in case the experimental data are ambiguous
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