In crystals where Z ′ > 1, the deviations in atomic positions for all asymmetric molecules are taken with respect to the first atom in the asymmetric unit cell. Geometry residuals with respect to the relative fractional coordinates are taken as unscaled deviations in the elements of Y s from the reference structure geometry, normalised by the square root of the number of residuals of this type (3(N r,s -1)),