A systematic benchmark of the ab initio Bethe-Salpeter equation approach for low-lying optical excitations of small organic molecules

摘要

The predictive power of the ab initio Bethe-Salpeter equation (BSE) approach,rigorously based on many-body Green's function theory but incorporatinginformation from density functional theory, has already been demonstrated forthe optical gaps and spectra of solid-state systems. Interest in photoactivehybrid organic/inorganic systems has recently increased, and so has the use ofthe BSE for computing neutral excitations of organic molecules. However, nosystematic benchmarks of the BSE for neutral electronic excitations of organicmolecules exist. Here, we study the performance of the BSE for the 28 smallmolecules in Thiel's widely-used time-dependent density functional theorybenchmark set [M. Schreiber et al. J. Chem. Phys. 128, 134110 (2008)]. Weobserve that the BSE produces results that depend critically on the mean-fieldstarting point employed in the perturbative approach. We find that thisstarting point dependence is mainly introduced through the quasiparticleenergies obtained at the intermediate GW step, and that with a judicious choiceof starting mean-field, singlet excitation energies obtained from BSE are inexcellent quantitative agreement with higher-level wavefunction methods. Thequality of the triplet excitations is slightly less satisfactory.