<i>Ab initio </i>calculations of photon dispersion in CdGa<sub>2</sub>Se<sub>4</sub>
By the method of density function the phonon states density and phonons dispersion in Brillouin zone is calculated. For A, B and E symmetry vibrations the moving of atons in the unit of cell is constructed. Optical phonons frequencies in the centre of Brillouin zone are in good agreement with experimentally obtained values from IR absorption and combinational scattering of light spectra. In a ab plane the crossing of dispersion curves of low-frequency optical phonons with the acoustic ones is observed.