Bethe-Salpeter equation without empty electronic states applied to charge-transfer excitations

We present an approach to compute optical absorption spectra of molecules and nanostructures from first principles, which is suitable for the study of large systems and gives access to spectra within a wide energy range. In this approach, the quantum Liouville equation is solved iteratively within first order perturbation theory, with a Hamiltonian containing a static self-energy operator. This is equivalent to solving the Bethe-Salpeter equation. Explicit calculations of single particle excited states and inversion of dielectric matrices are avoided using techniques based on Density Functional Perturbation Theory [1,2]. In this way, full absorption spectra may be obtained with a computational workload comparable to ground state Hartree-Fock calculations. Applications to the description of charge transfer excitations are presented. [1] D.Rocca, D.Lu and G.Galli (submitted) [2] H. Wilson, F. Gygi and G. Galli, Phys. Rev. B , 78, 113303, 2008;H. Wilson, D. Lu, F. Gygi and G. Galli, Phys. Rev. B, 79, 245106,2009.