(2.4.9) Gritsenko : A useful concept of Kohn-Sham DFT is the meaning of the energies of the Kohn-Sham orbitals. According to the Kohn-Sham analogue of Koopmans' theorem, the energy ε i of the occupied Kohn-Sham orbital φ i can be interpreted as approximate relaxed vertical potential I i of the primary ionization, ε i ≈ -I i . The quality of this approximation is better for the outer-valence Kohn-Sham orbitals, with equality for the HOMO. The deviation of ε i of the lower-lying Kohn-Sham orbitals from -I i is, primarily, due to the spectroscopically-averaged contributions from ionization of the corresponding "shake-up" satellites. The energies ε a of virtual Kohn-Sham orbitals φ a include the "excitonic" type particle-hole interaction; see contribution (2.2.3). The difference ε aε i therefore serves as a goodquality zero-order estimate of the corresponding excitation energy from TDDFT. With the electron affinity provided by the energy of the anionic Kohn-Sham HOMO, the Kohn-Sham orbitals deliver all the important one-electron quantities.