Assessing GW Approaches for Predicting Core Level Binding Energies.

Here we present a systematic study on the performance of different GW approaches: GW, GW with linearized quasiparticle equation (lin-GW), and quasiparticle self-consistent GW (qsGW), in predicting core level binding energies (CLBEs) on a series of representative molecules comparing to Kohn-Sham (KS) orbital energy-based results. KS orbital energies obtained using the PBE functional are 20-30 eV lower in energy than experimental values obtained from X-ray photoemission spectroscopy (XPS), showing that any Koopmans-like interpretation of KS core level orbitals fails dramatically. Results from qsGW lead to CLBEs that are closer to experimental values from XPS, yet too large. For the qsGW method, the mean absolute error is about 2 eV, an order of magnitude better than plain KS PBE orbital energies and quite close to predictions from ΔSCF calculations with the same functional, which are accurate within ∼1 eV. Smaller errors of ∼0.6 eV are found for qsGW CLBE shifts, again similar to those obtained using ΔSCF PBE. The computationally more affordable GW approximation leads to results less accurate than qsGW, with an error of ∼9 eV for CLBEs and ∼0.9 eV for their shifts. Interestingly, starting GW from PBE0 reduces this error to ∼4 eV with a slight improvement on the shifts as well (∼0.4 eV). The validity of the GW results is however questionable since only linearized quasiparticle equation results can be obtained. The present results pave the way to estimate CLBEs in periodic systems where ΔSCF calculations are not straightforward although further improvement is clearly needed.