(2.2.19) Gritsenko : The unique feature of the exchangecorrelation part of the local Kohn-Sham potential is the richness of the physical information on the local effects of electron correlation, as reflected in the shape of the potential. This can be favourably compared with the nonlocal Hartree-Fock and selfenergy potentials of the wave-function theory produced from the corresponding kernels. The shape of the latter potentials is "ruined" with singularities related to the orbital nodes. Contrary to this, the steps of the Kohn-Sham exchange-correlation potential meaningfully distinguish the local correlation effects in adjacent atomic and molecular shells with the corresponding "gauges" (see contribution (2.2.3)), while its integer discontinuity "jumps" signal occupation of (previously virtual) Kohn-Sham orbitals.