Tables for
Volume A
Space-group symmetry
Edited by M. I. Aroyo

International Tables for Crystallography (2016). Vol. A, ch. 1.6, pp. 114-125

Section Introduction

H. D. Flackb Introduction

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In small-molecule crystallography, it has been customary in crystal-structure analysis to make no use of the contribution of resonant scattering (otherwise called anomalous scattering and in older literature anomalous dispersion) other than in the specific area of absolute-structure and absolute-configuration determination. One may trace the causes of this situation to the weakness of the resonant-scattering contribution, to the high cost in time and labour of collecting intensity data sets containing measurements of all Friedel opposites and for a lack of any perceived or real need for the additional information that might be obtained from the effects of resonant scattering.

On the experimental side, the turning point came with the widespread distribution of area detectors for small-molecule crystallography, giving the potential to measure, at no extra cost, full-sphere data sets leading to the intensity differences between Friedel opposites hkl and [\overline{h}\overline{k}\overline{l}]. In 2015, the new methods of data analysis briefly presented here are in the stage of development and have not yet enjoyed widespread distribution, use and acceptance by the community. Flack et al. (2011[link]) and Parsons et al. (2012[link]) give detailed information on these calculations.


Flack, H. D., Sadki, M., Thompson, A. L. & Watkin, D. J. (2011). Practical applications of averages and differences of Friedel opposites. Acta Cryst. A67, 21–34.
Parsons, S., Pattison, P. & Flack, H. D. (2012). Analyzing Friedel averages and differences. Acta Cryst. A68, 736–749.

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