The spectral function (which is related to photoemission spectra) thus shows, for each spin, two peaks with the same spectral weight 1/2 -one for the removal of an electron (peak at ε 0 , which is the orbital energy), and one for the addition of a second electron (peak at ε 0 +U, with U the on-site electron-electron interaction), representing, respectively, the removal and addition energies, of an isolated atom with one electron.