Accuracy Assessment of <i>GW</i> Starting Points for Calculating Molecular Excitation Energies Using the Bethe–Salpeter Formalism

The performance of the Bethe–Salpeter equation (BSE) approach for the first-principles computation of singlet and triplet excitation energies of small organic, closed-shell molecules has been assessed with respect to the quasiparticle energies used on input, obtained at various levels of <i>GW</i> theory. In the corresponding <i>GW</i> computations, quasiparticle energies have been computed for <i>all</i> orbital levels by means of using full spectral functions. The assessment reveals that, for valence excited states, quasiparticle energies obtained at the levels of eigenvalue-only self-consistent (ev<i>GW</i>) or quasiparticle self-consistent theory (qs<i>GW</i>) are required to obtain results of comparable accuracy as in time-dependent density-functional theory (TDDFT) using a hybrid functional such as PBE0. In contrast to TDDFT, however, the BSE approach performs well not only for valence excited states but also for excited states with Rydberg or charge-transfer character. To demonstrate the applicability of the BSE approach, computation times are reported for a set of aromatic hydrocarbons. Furthermore, examples of computations of ordinary photoabsorption and electronic circular dichroism spectra are presented for (C₆₀)₂ and C₈₄, respectively