@article{214,
  keywords = {paper},
  author = {P. Romaniello and D Sangalli and J Berger and Francesco Sottile and L. Molinari and Lucia Reining and G Onida},
  title = {Double excitations in finite systems},
  abstract = {Time-dependent density-functional theory \textquoterightTDDFT\textquoteright 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\textendashSalpeter 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.},
  year = {2009},
  journal = {Journal of Chemical Physics},
  volume = {130},
  pages = {044108},
  url = {http://dx.doi.org/10.1063/1.3065669},
  doi = {10.1063/1.3065669},
}