Adiabatic and nonadiabatic phonon dispersion in a Wannier function approach

We develop a first-principles scheme to calculate adiabatic and nonadiabatic phonon frequencies in the full Brillouin zone. The method relies on the stationary properties of a force-constant functional with respect to the first-order perturbation of the electronic charge density and on the localization of the deformation potential in the Wannier function basis. This allows for calculation of phonon-dispersion curves free from convergence issues related to Brillouin-zone sampling. In addition our approach justifies the use of the static screened potential in the calculation of the phonon linewidth due to decay in electron-hole pairs. We apply the method to the calculation of the phonon dispersion and electron-phonon coupling in MgB2 and CaC 6. In both compounds we demonstrate the occurrence of several Kohn anomalies, absent in previous calculations, that are manifest only after careful electron- and phonon-momentum integration. In MgB2, the presence of Kohn anomalies on the E2g branches improves the agreement with measured phonon spectra and affects the position of the main peak in the Eliashberg function. In CaC6 we show that the nonadiabatic effects on in-plane carbon vibrations are not localized at zone center but are sizable throughout the full Brillouin zone. Our method opens perspectives in large-scale first-principles calculations of dynamical properties and electron-phonon interaction. © 2010 The American Physical Society.