Mixed-space approach for calculation of vibration-induced dipole-dipole interactions

By explicitly taking into account the effects of vibration-induced dipole-dipole interactions between periodic supercells, we derive an efficient formulation to calculate the phonon frequencies of an ionic crystal. We demonstrate that the vibration-induced dipole-dipole interactions lead to a constant contribution to the interatomic force constant in real space. It recovers the result of Cochran and Cowley at the long wavelength limit. Using MgO as the prototype, we demonstrate that a 16-atom 2×2×2 supercell of the primitive unit cell is sufficient to obtain the phonon dispersions when the dipole-dipole interactions are considered. We find that not properly taking into account the dipole-dipole interaction leads to oscillations along the (0,0,q) direction for the longitudinal optical phonon dispersion in a 128-atom elongated 1×1×16 supercell with a cubic structure.