where the overbar indicates the coupling strength integrated paircorrelation function. The potential vhole xc , the derivative of the ρ functions under the integral, represents the universal interaction (for both occupied and virtual Kohn-Sham orbitals) with the exchange-correlation hole of the unit charge. In turn, the potential vresp , the derivative of the pair-correlation function ḡxc , exhibits the spatial step-like structure, with the individual steps distinguishing various atomic and molecular electron shells. (2.2.4) Baerends : The Kohn-Sham method is often cited as the method that made DFT a feasible computational method by offering a decent approximation to (a large part of) the kinetic energy. The latter proved too hard to obtain as a density functional. But more importantly, the Kohn-Sham method has provided DFT with an orbital model. This has greatly facilitated its acceptance in the computational chemistry community. After initial reservations about the Kohn-Sham orbitals ("they are only there to build the density"), it has become evident that these orbitals are not inferior to or more approximate than the Hartree-Fock orbitals, but on the contrary are even more suitable for the qualitative and semiquantitative molecular-orbital (MO) theories of chemistry. If the exact Kohn-Sham orbitals and orbital energies could be obtained, this would be evident.