Accidental diffraction extinction from equal atomic scattering factors

A vanishing crystal basis structure factor can depend on approximate equality of atomic scattering factors, rather than on an exact space group condition. For two inequivalent site classes, an amplitude $F=C(f_A-f_B)$ vanishes when $f_A=f_B$ but becomes weakly observable if bonding electron densities or thermal displacements differ. Chemical identity alone does not imply identical local atomic scattering factors. The diffraction intensity is quadratic in the small difference.