The importance of weak reflections in resolving the centrosymmetric-noncentrosymmetric ambiguity: a cautionary tale

The weak diffraction intensities are crucially important in determining whether a crystal structure has a real or only an approximate, center of symmetry; if these intensities are deleted from the data set, an intelligent conclusion may be impossible. In addition, statistical distribution tests, if based only on the stronger intensities, may be strongly biased toward a noncentrosymmetric indication. In one recently published structure determination [Cotton & Fanwick (1980), Acta Cryst. B36, 457-459] a distribution test led to the assignment of the noncentrosymmetric space group Cc to a structure which can be better described and refined in the centrosymmetric space group C2/c.