First-principles evaluation of dynamical response and plasmon dispersion in metals

We report an ab initio evaluation of the dynamical density-response function for real Al and Na. The method we employ is a generalization of the Dalgamo-Lewis scheme of perturbation theory to systems with a continuous spectrum and to dynamical problems. The crystal lattice is found to lower the plasmon frequency of Al for large wave vectors by as much as 4 eV. For this metal, agreement with experiment is excellent in the time-dependent local-density approximation for exchange correlation. The ``anomalous'' dispersion observed for Na for large wave vectors is assigned to subtler exchange-correlation effects.