Kohn-Sham wavefunction, definition

The KS equations are obtained by differentiating the energy with respect to the KS molecular orbitals, analogously to the derivation of the Hartree-Fock equations, where differentiation is with respect to wavefunction molecular orbitals (Section 5.2.3.4). We use the fact that the electron density distribution of the reference system, which is by decree exactly the same as that of the ground state of our real system (see the definition at the beginning of the discussion of the Kohn-Sham energy), is given by (reference [9])... 

By exchange, we mean the density functional definition of exchange, in which the wavefunction is a Slater determinant whose density is the exact density of the interacting system, and which minimizes the energy of the non-interacting system in the Kohn-Sham external potential, iv,a=o- Another useful concept is the pair distribution function, defined as[14]... 

Correlation energy is ultimately a convenient container for all that escapes from first-order coulombic and polarization terms. See Bickelhaupt, F. M. Baerends, E. J. Kohn-Sham density functional theory predicting and understanding chemistry, Reviews in Computational Chemistry, Vol. 15, edited by K. B. Lipkowitz and D. B. Boyd, 2000, Wiley-VCH, New Yorkp.ll Is electron correlation (not defined in a statistical sense, but according to either the quantum chemical or the DFT definition) a true physical phenomenon It is a man-made concept, related to the introduction of a convenient trial wavefunction, that is useful for our communication and understanding. ... 

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