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For non-centrosymmetric structures in space group P1 containing a centrosymmetric substructure, analytical expressions have been obtained for various functions of the diffraction intensity of Friedel opposites. These functions are the average intensity of Friedel opposites, the mean difference in intensity of Friedel opposites and the mean-square difference in intensity of Friedel opposites. A Bijvoet intensity ratio is defined for the evaluation of resonant-scattering effects in non-centrosymmetric and pseudo-centrosymmetric structures. Analysis of these expressions confirms that both resonant and non-resonant atoms are necessary to produce differences in intensity between Friedel opposites and also shows that in some circumstances atoms may lie on a centrosymmetric substructure without diminishing the Bijvoet intensity ratio. The effects of the real component of resonant scattering, of the variation of the scattering factors with sin θ/λ, of isotropic atomic displacement parameters, of a crystal twinned by inversion, of atoms in special positions and of weak reflections are considered. Software is available for the evaluation of the Bijvoet intensity ratio.

Supporting information

xls

Microsoft Excel (XLS) file https://doi.org/10.1107/S0108767307002802/sh5056sup1.xls
Spreadsheet in Excel format (original version)

ods

Open Spreadsheet Text (ODS) file https://doi.org/10.1107/S0108767307002802/sh5056sup2.ods
Spreadsheet in ODS format (original version)

xml

Extensible Markup Language (XML) file https://doi.org/10.1107/S0108767307002802/sh5056sup3.xml
Spreadsheet in xml format (original version)

xls

Microsoft Excel (XLS) file https://doi.org/10.1107/S0108767307002802/sh5056sup4.xls
Spreadsheet in Excel format (new version supplied by the author on 8 May 2008)

ods

Open Spreadsheet Text (ODS) file https://doi.org/10.1107/S0108767307002802/sh5056sup5.ods
Spreadsheet in ODS format (new version supplied by the author on 8 May 2008)

xml

Extensible Markup Language (XML) file https://doi.org/10.1107/S0108767307002802/sh5056sup6.xml
Spreadsheet in xml format (new version supplied by the author on 8 May 2008)


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