Double excitations in finite systems

TitleDouble excitations in finite systems
Publication TypePalaiseau Article


Author Address

Laboratoire des Solides Irradiés UMR 7642, CNRS-CEA/DSM, École Polytechnique, F-91128 Palaiseau,France
Istituto Nazionale per la Fisica della Materia, CNISM, and Dipartimento di Fisica, Università di Milano, Via Celoria 16, I-20133 Milano, Italy
European Theoretical Spectroscopy Facility (ETSF), F-91128 Palaiseau, France

Romaniello, P, Sangalli, D, Berger, JA, Sottile, F, Molinari, LG, Reining, L, Onida, G
Year of Publication2009
JournalJournal of Chemical Physics

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 fxc 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, 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.

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