Koopmans meets Bethe-Salpeter: Excitonic optical spectra from GW-free simulations.

The Bethe–Salpeter equation (BSE) can be applied to compute from first-principles optical spectra that include the effects of screened electron–hole interactions. As input, BSE calculations require single-particle states, quasiparticle energy levels, and the screened Coulomb interaction, which are typically obtained with many-body perturbation theory, whose cost limits the scope of possible applications. This work tries to address this practical limitation, instead deriving spectral energies from Koopmans-compliant functionals and introducing a new methodology for handling the screened Coulomb interaction. The explicit calculation of the W matrix is bypassed via a direct minimization scheme applied on top of a maximally localized Wannier function basis. We validate and benchmark this approach by computing the low-lying excited states of the molecules in Thiel’s set and the optical absorption spectrum of a C60 fullerene. The results show the same trends as quantum chemical methods and are in excellent agre...