A QUANTUM-CHEMICAL VIEW OF DENSITY-FUNCTIONAL THEORY

A comparison is made between traditional quantum chemical approaches t o the electron correlation problem and the one taken in density functi onal theory (DFT). Well-known concepts of DFT, such as the exchange-co rrelation energy E-xc = integral rho(r) epsilon(xc)(r) dr and the exch ange-correlation potential upsilon(xc)(r) are related to electron corr elation as described in terms of density matrices and the conditional amplitude (Fermi and Coulomb holes). The Kohn-Sham one-electron or orb ital model of DFT is contrasted with Hartree-Fock, and the definitions of exchange and correlation in DFT are compared with the traditional ones. The exchange-correlation energy density epsilon(xc)(r) is decomp osed into kinetic and electron-electron potential energy components, a nd a practical way of calculating these from accurate wave functions i s discussed, which offers a route to systematic improvement. upsilon(x c)(r) is likewise decomposed, and special features (bond midpoint peak , various types of step behavior) are identified and related to electr onic correlation.