  Koopmans-compliant (KC) functionals have been shown to provide accurate
spectral properties through a generalized condition of piece-wise linearity of
the total energy as a function of the fractional addition/removal of an
electron to/from any orbital. We analyze the performance of different KC
functionals on the GW100 test-set, comparing the ionization potentials (as
opposite of the energy of the highest occupied orbital) of these 100 molecules
to those obtained from CCSD(T) total energy differences, and experimental
results, finding excellent agreement with a mean absolute error of 0.20 eV for
the KIPZ functional, that is state-of-the-art for both DFT-based calculations
and many-body perturbation theory. We highlight similarities and differences
between KC functionals and other electronic-structure approaches, such as
dielectric-dependent hybrid functionals and G$_0$W$_0$, both from a theoretical
and from a practical point of view, arguing that Koopmans-compliant potentials
can be considered as a local and orbital-dependent counterpart to the
electronic GW self-energy, albeit already including approximate vertex
corrections.
