Intensity statistics of Friedel opposites

A previous analysis of the average intensity and mean-square intensity difference of Friedel opposites, confined to the space group P1 [Flack & Shmueli (2007). Acta Cryst. A63, 257-265], is here extended to all the non-centrosymmetric space groups. The present analysis presumes purely non-centrosymmetric content of the unit cell. An important result of this study is that the average intensity and mean-square intensity difference of Friedel opposites have the same values for all the non-centrosymmetric space groups as those previously obtained for the triclinic space group P1. The ratios of average intensity and root-mean-square intensity difference to their triclinic equivalents were derived and exemplified for general as well as for special reflections. For the latter, enhancements were obtained which are shown to be due to those of average intensity and not to a mechanism related to Friedel opposites being explicitly considered.