Microscopic Origin of Infrared Activity in Graphite

We investigate the phonon dispersion of the graphite within a generalized bond-charge model with emphasis on the microscopic origin of the infrared active mode. The resulting dispersion is in good agreement with those obtained by the experiments which include the full optical spectra obtained by the recent x-ray scattering. The computed strengths of the infrared peak are found to be comparable to the experimental values. We show that the dipole moment responsible for the infrared activity is induced by the interlayer Coulomb interaction between $\pi$- bond electrons.