In our recent experimental charge density studies, we noticed residual density close to hydrogen positions that could be modelled as anisotropic displacement. To justify this treatment we collected neutron data for two benchmark structures and compared the derived anisotropic hydrogen displacement parameters to our refined parameters against the X-ray data, results obtained from the SHADE-server¹, the software APD-Toolkit² and from HARt³. The smallest deviations from the neutron values was achieved by the refinement of anisotropic hydrogen displacement parameters against the X-ray data. With that, the refinement of bond-directed quadrupole parameters turned out to be important. The main discrepancies were observed for hydrogen atoms that are attached to carbon atoms that were refined with Gram-Charlier coefficients. Furthermore, we investigated also the free refinement of the C-H bond distances after anisotropic refinement of the hydrogens. While HARt yields the most reliable C-H bond distances, it overestimated the hydrogen anisotropic displacement parameters.⁴

¹Madsen, A. Ø. & Hoser, A. A. (2014). J. Appl. Cryst. 47, 2100–2104.

²Lübben, J., Bourhis, L. J. & Dittrich, B. (2015). J. Appl. Cryst. 48, 1785–1793.

³Fugel, M., Jayatilaka, D., Hupf, E., Overgaard, J., Hathwar, V. R., Macchi, P., Turner, M. J., Howard, J. A. K., Dolomanov, O. V., Puschmann, H., Iversen, B. B., Bürgi, H.-B. & Grabowsky, S. (2018). IUCrJ. 5, 32–44.

⁴Köhler, C.; Lübben, J.; Hoffmann, C., Krause, L.; Herbst-Irmer, R, & Stalke, D. (2019). Acta Cryst. B75, in production.