1The Flack parameter estimated in this way is referred to as the Hooft parameter and given the symbol y.

2A referee to this paper pointed out that reverse-beam measurements can be made for low-angle reflections on a four-circle diffractometer with an area detector by measuring 360° [varphi]-scans at [chi] = +90 and -90°. In this case Friedel pairs fall on the same pixels of the detector eliminating another source of possible systematic error. The use of data collected in this way for measuring Qobs(h) is a very interesting avenue for further work.

3Supplementary data for this paper are available from the IUCr electronic archives (Reference: GP5062 ). Services for accessing these data are described at the back of the journal.