Charge flipping at work: a case of pseudosymmetry

Charge flipping (CF) is an amazingly simple structure solution method that uses single-crystal X-ray diffraction data. It is truly ab initio, no preliminary information on atom types, chemical composition, or space group symmetry is required. The algorithm is iterative and alternates between real and reciprocal spaces. Its simplest version only changes (flips) the sign of the electron density below a threshold, while in reciprocal space, it prescribes the moduli of observed structure factors. In this communication, we apply the algorithm in practice. The selected example presents a whole range of difficulties: it is large, contains only light atoms, is noncentrosymmetric, and shows a particularly awkward pseudosymmetry. To solve it with any of the traditional methods requires many hours of computer time, followed by a day of expert's handwork to find missing and to remove spurious atoms. In contrast, the CF algorithm provides the complete structure in a few seconds and without human intervention. It is also remarkable that the success rate is 100%, that is, any starting point in the high-dimensional phase space leads to the solution. The treatment of translational pseudosymmetry is obviously a favorable case. Similar resistant structures with pseudosymmetries or ambiguous space groups are the practical applications where the CF method could well complement standard software procedures.