The self-consistent GWΓ method satisfies the Ward-Takahashi identity (i.e., the gauge invariance or the local charge continuity) for arbitrary energy (ω) and momentum (q) transfers. Its self-consistent firstprinciples treatment of the vertex Γ = Γ v or Γ W is possible to first order in the bare (v) or dynamicallyscreened (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 .
1The quasiparticle (QP) equation method in many-body perturbation theory [1] is powerful for simultaneously determining the photoemission (or inverse photoemission) spectra (PES), i.e., QP energy spectra, and QP wave functions of target materials from first-principles. In this method, we expand the skeleton diagrams, i.e., the diagrams drawn with the full Green's function lines, for the self-energy in terms of the electron-electron Coulomb interaction v, and solve the QP equation, which is equivalent to the Dyson equation, as a self-consistent (SC) eigenvalue problem. The Hartree-Fock (HF) approach provides the first-order approximation. In Hedin's set of equations[1] known as the GWΓ approach, the exchange-correlation part of the self-energy is expressed aswhere G σ and Γ σ are the one-particle Green's function and the vertex function (σ is the spin index), respectively, and W = (1 − vP) −1 v represents the dynamically screened Coulomb interaction (P = −i σ G σ G σ Γ σ is the polarization function). The simplest approximation is to assume Γ σ = 1, which is called the GW approximation.It is well known that the SC GW method usually overestimates the energy gap [2, 3], while the one-shot GW approach (G 0 W 0 ) using the Kohn-Sham (KS) wave functions and eigenvalues[4] results in a better energy gap. However, quite recently, it has been pointed out that the photoabsorption spectra (PAS) for small molecules obtained by solving the Bethe-Salpeter equation (BSE) [5, 6] using G 0 W 0 are often significantly redshifted by about 1 eV [7,8]. The use of the Heyd-Scuseria-Ernzerhof (HSE) functional or the SC GW calculation (hereafter referred to as GW) improves the results, but they are not perfect [8,9]. For a spin-polarized sodium atom (Na) and trimer (Na 3 ), G 0 W 0 + BSE is extremely bad, although the G 0 W 0 QP energies are reasonably good [10]. The calculated and experimental [11] optical gaps for Na are 1.32 eV and 2.10 eV, respectively, and the calculated and experimental [12] PAS for Na 3 are shown in Fig. 1. These calculated results are far off from the experimental data [13].Here, we develop a GWΓ method, which involves a SC treatment of the vertex Γ = Γ v or Γ W and satisfies the Ward-Takahashi identity [14][...