The problem of generation of the selection rules for a transition between Bloch states at any point of the Brillouin zone in crystals is equivalent to the problem of the decomposition of Kronecker products of two representations (reps) of a space group into irreducible components (the full group method). This problem can be solved also by the subgroup method where small reps of little groups are used. In this article, we propose the third method of the selection rules' generation which is formulated in terms of projective reps of crystal point groups. It is based on a well known relation between small irreducible reps (irreps) of little space groups and projective irreps of corresponding little co-groups. The proposed procedure is illustrated by calculations of the Kronecker products for different irreps at the W point of the Brillouin zone for the nonsymmorphic space group O⁷_h being one of the most complicated space groups for the selection rules' generation. As an example, the general procedure suggested is applied to obtain the selection rules for direct and phonon-assisted electrical dipole transitions between some states in crystals with the space group O⁷_h. One of the authors (V.P.S.) acknowledges the support of Ministere de la Recherche (France).

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