Assessing GW approaches for predicting core level binding energies

Abstract

Here we present a systematic study on the performance of different GW approaches: G(0)W(0), G(0)W(0) with linearized quasiparticle equation (lin-G(0)W(0)), 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 Delta SCF calculations with the same functional, which are accurate within similar to 1 eV. Smaller errors of similar to 0.6 eV are found for qsGW CLBE shifts, again similar to those obtained using Delta SCF PBE. The computationally more affordable G(0)W(0) approximation leads to results less accurate than qsGW, with an error of similar to 9 eV for CLBEs and similar to 0.9 eV for their shifts. Interestingly, starting G(0)W(0) from PBEO reduces this error to similar to 4 eV with a slight improvement on the shifts as well (similar to 0.4 eV). The validity of the G(0)W(0) 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 Delta SCF calculations are not straightforward although further improvement is clearly needed.