title: Double excitations in finite systems.
abstract: Time-dependent density-functional theory (TDDFT) is widely used in the study of linear response properties of finite systems. However, there are difficulties in properly describing excited states, which have double- and higher-excitation characters, which are particularly important in molecules with an open-shell ground state. These states would be described if the exact TDDFT kernel were used; however, within the adiabatic approximation to the exchange-correlation (xc) kernel, the calculated excitation energies have a strict single-excitation character and are fewer than the real ones. A frequency-dependent xc kernel could create extra poles in the response function, which would describe states with a multiple-excitation character. We introduce a frequency-dependent xc kernel, which can reproduce, within TDDFT, double excitations in finite systems. In order to achieve this, we use the Bethe-Salpeter equation with a dynamically screened Coulomb interaction W(omega), which can describe these excitations, and from this we obtain the xc kernel. Using a two-electron model system, we show that the frequency dependence of W does indeed introduce the double excitations that are instead absent in any static approximation of the electron-hole screening.
authors.affiliation: Laboratoire des Solides Irradies UMR 7642, CNRS-CEA/DSM, Ecole Polytechnique, F-91128 Palaiseau, France. pina.romaniello@polytechnique.edu
authors.forename: P
authors.initials: P
authors.lastname: Romaniello
authors.forename: D
authors.initials: D
authors.lastname: Sangalli
authors.forename: J A
authors.initials: JA
authors.lastname: Berger
authors.forename: F
authors.initials: F
authors.lastname: Sottile
authors.forename: L G
authors.initials: LG
authors.lastname: Molinari
authors.forename: L
authors.initials: L
authors.lastname: Reining
authors.forename: G
authors.initials: G
authors.lastname: Onida