HUG and SQUEEZE: using CRYSTALS to incorporate resonant scattering in the SQUEEZE structure-factor contributions to determine absolute structure.

The resonant-scattering contributions to single-crystal X-ray diffraction data enable the absolute structure of crystalline materials to be determined. Crystal structures can be determined even if they contain considerably disordered regions because a correction is available via a discrete Fourier transform of the residual electron density to approximate the X-ray scattering from the disordered region. However, the corrected model cannot normally account for resonant scattering from atoms in the disordered region. Straightforward determination of absolute structure from crystals where the strongly resonantly scattering atoms are not resolved has therefore not been possible. Using an approximate resonant-scattering correction to the X-ray scattering from the disordered regions, we have developed and tested a procedure (HUG) to recover the absolute structure using conventional Flack x refinement or other post-refinement determination methods. Results show that in favourable cases the HUG method works well and the absolute structure can be correctly determined. It offers no useful improvement in cases where the original correction for the disordered region scattering density is problematic, for example, when a large fraction of the scattering density in the crystal is disordered, or when voids are not occupied equally by the disordered species. Crucially, however, if the approach does not work for a given structure, the statistics for the absolute structure measures are not improved, meaning it is unlikely to lead to misassignment of absolute structure.