Theoretical descriptions of the spectrum of electronic excitations in real
metals have not yet reached a fully predictive, "first-principles" stage. In
this paper we begin by presenting brief highlights of recent progress made in
the evaluation of dynamical electronic response in metals. A comparison between
calculated and measured spectra - we use the loss spectra of Al and Cs as test
cases - leads us to the conclusion that, even in "weakly-correlated" metals,
correlation effects beyond mean-field theory play an important role.
Furthermore, the effects of the underlying band structure turn out to be
significant. Calculations which incorporate the effects of both dynamical
correlations and band structure from first principles are not yet available. As
a first step towards such goal, we outline a numerical algorithm for the
self-consistent solution of the Dyson equation for the one-particle Green's
function. The self-energy is evaluated within the shielded-interaction
approximation of Baym and Kadanoff. Our method, which is fully conserving, is a
finite-temperature scheme which determines the Green's function and the
self-energy at the Matsubara frequencies on the imaginary axis. The analytical
continuation to real frequencies is performed via Pade approximants. We present
results for the homogeneous electron gas which exemplify the importance of
many-body self-consistency.Comment: 32 pages, 6 figures; "Fifty Years of the Correlation Problem",
invited paper, to be published in Mol.Phy