Crystallographic and electronic structures of binary compounds GaSe and InSe, known to exist in several polytype modifications, are discussed in comparison with those of chalcopyrite-type CuInSe2, CuGaSe2, and their solid solutions. The results are obtained within the density-functional theory (DFT), using two calculation codes, WIEN2k and VASP. The van der Waals interaction, important in stabilizing the structure of the layer compounds GaSe and InSe, was taken into account in the VASP calculation along the scheme of Grimme. For obtaining more realistic electronic band structures than is possible with the conventional generalized gradient approximation (GGA), the modified Becke–Johnson (mBJ) exchange–correlation (XC) potential was used, as implemented in the WIEN2k code. The results of two computer codes were compared in what concerns the optimization of the ground-state structures. The similarities in electronic structures of the systems studied are discussed under an angle of general trends proper to the compounds of Cu–(In,Ga)–Se composition.