Charge flipping is an increasingly popular structure solution method based on an iterative application of constraints in both direct and reciprocal space. The output of the structure-solution process is a scattering density in the unit cell of the crystal. It is very successful, if good-quality single-crystal diffraction data are available. The intrinsic problem of the structure solution from powder diffraction data is the reflection overlap, as a result of which the intensities of individual reflections can be extracted only approximately. Consequently, the scattering density obtained with these approximate intensities is often of mediocre quality, even if the phases are known. This problem can be circumvented by combining the basic charge-flipping algorithm with a histogram matching procedure. The histogram of a structure can be satisfactorily estimated from the chemical composition only, without the knowledge of the crystal structure, and as such provides additional external information. Modifying an intermediate electron density to fit the expected histogram results in a change of its Fourier spectrum, and this improved spectrum can be used to repartition the intensities of the overlapped reflections.

The method has been applied to extracted intensities of several powder data sets [1], where the overlap was determined based on the angular separation of the reflections in the pattern, and the overlapped reflections were repartitioned according to the intensity ratios of the histogram-matched density. Although the method works well, it is clearly not exploiting all the information contained in the diffraction pattern, because the distribution of the intensities within one overlap group is not used in the process. Therefore a method was generalized to use the full powder diffraction pattern during the histogram-matching procedure.

The charge flipping combined with histogram matching is a powerful method especially in cases of large unit cells with strong systematic overlap. On the other hand, it is limited by the requirement of atomic-resolution data, i.e. effective resolution of about dmin=1.2 or better. That means that for structures with larger unit cells very good experimental data is required, usually data obtained with synchrotron radiation. 

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