(2.2.5) Krylov : The orbital picture of Kohn-Sham DFT is indeed of great importance. With the exact functional, the energy of the highest occupied Kohn-Sham orbitals become exact ionization energies (as per Janak's theorem). Numerical investigations show that the shapes of the KS orbitals in cases when their IEs are close to the exact IE (such as when tuning range-separation parameter to make Koopmans IE to match Delta SCF IE) become similar to the shapes of the Dyson orbitals . Interestingly, the energies of lower-lying Kohn-Sham orbitals provide surprisingly accurate approximations to the exact many-body ionization energies (IEs) (when used with appropriate DFAs), which can be understood by analysing the curvature of the total Kohn-Sham energy with respect to the occupation numbers. This endows the theory with the ability to provide physically relevant quantities -for example, Dyson orbitals enter the expressions for photoionization/photodetachment cross-sections and can even be reconstructed from experimental data. Moreover, the orbitals provide a link between many-body theories and DFT -for example, one can judge the quality of a particular DFA by how well the shapes and energies of the Kohn-Sham orbitals agree with those from high-level many-body calculations (e.g., equation-of-motion coupled-cluster theory). These ideas are already exploited in optimally-tuned range-separated DFAs. But, perhaps more opportunities exist for using ab initio Dyson orbitals to build better DFAs? Specifically, the electronic states of small metal clusters are bunched in shells. These shells are experimentally observed in the variations of polarizabilities, ionization energies, and electron affinities -to name a few characteristic observables. Kohn-Sham orbitals, as approximations to Dyson orbitals, reflect these shell structures in a large variety of free and ligand-stabilized clusters. Thus, the now common concept of superatoms in chemistry is based almost exclusively on Kohn-Sham calculations and the corresponding canonical Kohn-Sham orbitals.