Comparison of Green's functions for transition metal atoms using self-energy functional theory and coupled-cluster singles and doubles (CCSD)

We demonstrate in the present study that self-consistent calculations based on the self-energy functional theory (SFT) are possible for the electronic structure of realistic systems in the context of quantum chemistry. We describe the procedure of a self-consistent SFT calculation in detail and perform the calculations for isolated $3 d$ transition metal atoms from V to Cu as a preliminary study. We compare the one-particle Green's functions (GFs) obtained in this way and those obtained from the coupled-cluster singles and doubles (CCSD) method. Although the SFT calculation starts from the spin-unpolarized Hartree--Fock (HF) state for each of the target systems, the self-consistency loop correctly leads to degenerate spin-polarized ground states. We examine the spectral functions in detail to find their commonalities and differences among the atoms by paying attention to the characteristics of the two approaches. It is demonstrated via the two approaches that calculations based on the density functional theory (DFT) can fail in predicting the orbital energy spectra for spherically symmetric systems. It is found that the two methods are quite reliable and useful beyond DFT.