Abstract
The unit cell group and the factor group are basic to the theoretical analysis of the vibrations, or electronic transitions, in crystals in the k = 0 approximation. Yet some confusion exists in the literature concerning the definitions of these groups. This paper examines the literature definitions, draws attention to inconsistencies in the literature, and describes the unit cell group and the factor group for the specific cases of the symmorphic space group, P2 / m, and the nonsymmorphic space groups P21 / m and P21 / c.