Pseudo-merohedral twinning. The treatment of overlapped data

Expressions which enable data to be `corrected' for twinning are examined. If the volume ratio of the twin individuals is known, structure factors may be calculated and used in the normal way. Particular care must, however, be taken with weighting. For least-squares refinement or for comparing observed and calculated intensities, overlapped data can be used directly in the normal Kennicott procedure which initially requires a knowledge of the volume ratio, but can refine it. Alternatively the data can be modified in such a way that calculations are independent of the volume ratio. All three methods have been successfully applied to overlapped data from Para Red (1-p-nitrobenzeneazo-2-naphthol).