Phonons in HgSe and AgGaSe2 crystals from first-principle calculations

Using the ab initio local density approximation with ultrasoft pseudopotentials, the Hellmann-Feymnan forces for HgSe and AgGaSe2, have been calculated in 2 x 2 x 2 and 2 x 2 x I supercells, respectively. Applying the direct method the phonon dispersion relations have been derived. The LO/TO splitting of optic phonon branches have been found from effective charges. The calculated phonon dispersion curves are in very good agreement with the results of the coherent inelastic neutron scattering data.