Description:
<jats:sec><jats:label /><jats:p>The Koopmans’ theorem is a statement about the interpretation of Hartree–Fock eigenstates with several important generalizations. They allow for the extraction of excited electronic states starting from the ground state density correlators. Here, the theorem is applied to the computation of doubly excited states, that is, excited states containing a significat amount of doubly excited configurations. They are known to be difficult for time‐dependent density functional theory (TDDFT) and for the equation‐of‐motion (EOM) class of methods in general. Using two molecular systems <jats:italic>trans</jats:italic>‐butadiene and anthracene as examples, it is demonstrated that both, the self‐energy corrections and the corrections due to double electron transitions are important. Finally, the applicability of the extended Koopmans’ theorem (EKT) for the determination of spectral properties from the Matsubara Green's function (GF) approach and the Bethe–Salpeter equation are demonstrated.</jats:p></jats:sec>