title: Beyond time-dependent exact exchange: the need for long-range correlation.
abstract: In the description of the interaction between electrons beyond the classical Hartree picture, bare exchange often yields a leading contribution. Here we discuss its effect on optical spectra of solids, comparing three different frameworks: time-dependent Hartree-Fock, a recently introduced combined density-functional and Green's function approaches applied to the bare exchange self-energy, and time-dependent exact exchange within time-dependent density-functional theory (TD-EXX). We show that these three approximations give rise to identical excitonic effects in solids; these effects are drastically overestimated for semiconductors. They are partially compensated by the usual overestimation of the quasiparticle band gap within Hartree-Fock. The physics that lacks in these approaches can be formulated as screening. We show that the introduction of screening in TD-EXX indeed leads to a formulation that is equivalent to previously proposed functionals derived from many-body perturbation theory. It can be simulated by reducing the long-range part of the Coulomb interaction: this produces absorption spectra of semiconductors in good agreement with experiment.
authors.affiliation: European Theoretical Spectroscopy Facility (ETSF), Laboratoire des Solides Irradiés, UMR 7642, CNRS-CEA/DSM, Ecole Polytechnique, F-91128 Palaiseau, France. fabien.bruneval@polytechnique.fr
authors.forename: Fabien
authors.initials: F
authors.lastname: Bruneval
authors.forename: Francesco
authors.initials: F
authors.lastname: Sottile
authors.forename: Valerio
authors.initials: V
authors.lastname: Olevano
authors.forename: Lucia
authors.initials: L
authors.lastname: Reining