GW{\Gamma} + Bethe-Salpeter equation approach for photoabsorption spectra: Importance of self-consistent GW{\Gamma} calculations in small atomic systems

The self-consistent GW{\Gamma} method satisfies the Ward-Takahashi identity (i.e., the gauge invariance or the local charge continuity) for arbitrary energy ($\omega$) and momentum ($\mathbf{q}$) transfers. Its self-consistent first-principles treatment of the vertex $\mathrm{\Gamma }={\mathrm{\Gamma }}_v$ or ${\mathrm{\Gamma }}_W$ is possible to first order in the bare ($v$) or dynamically-screened ($W$) Coulomb interaction. It is developed within a linearized scheme and combined with the Bethe-Salpeter equation (BSE) to accurately calculate photoabsorption spectra (PAS) and photoemission (or inverse photoemission) spectra (PES) simultaneously. The method greatly improves the PAS of Na, Na${}_3$, B${}_2$, and C${}_2$H${}_2$ calculated using the standard one-shot $G_0{W}_0$ + BSE method that results in significantly redshifted PAS by 0.8-3.1 eV, although the PES are well reproduced already in $G_0{W}_0$.