Benchmarking the Bethe-Salpeter Formalism on a Standard Organic Molecular Set

We perform benchmark calculations of the Bethe Salpeter vertical excitation energies for the set of 28 molecules constituting the well-known Thiel's set, complemented by a series of small molecules representative of the dye chemistry field. We show that Bethe Salpeter calculations based on a molecular orbital energy spectrum obtained with non-self-consistent G(0)W(0) calculations starting from semilocal DFT functionals dramatically underestimate the transition energies. Starting from the popular PBEO hybrid functional significantly improves the results even though this leads to an average -0.59 eV redshift compared to reference calculations for Thiel's set. It is shown, however, that a simple self-consistent scheme at the GW level, with an update of the quasiparticle energies, not only leads to a much better agreement with reference values, but also significantly reduces the impact of the starting DFT functional. On average, the Bethe Salpeter scheme based on self-consistent GW calculations comes close to the best timedependent DFT calculations with the PBEO functional with a 0.98 correlation coefficient and a 0.18 (0.25) eV mean absolute deviation compared to TD-PBEO (theoretical best estimates) with a tendency to be red-shifted. We also observe that TD-DFT and the standard adiabatic Bethe Salpeter implementation may differ significantly for states implying a large multiple excitation character.