After the local-density approximation and the very successful generalized gradient approximations, it is often stated that using an exact exchange (EXX) functional is the next step up our ladder of DFT approximations. It would notably solve the self-interaction problem that plagues DFT. It only leaves the much smaller correlation energy to be treated with some approximate functional.

We will argue that the EXX approach cannot be considered a step forward compared to the GGAs. It reintroduces a very important flaw present in the Hartree-Fock approximation. Arguably the great success of DFT is a consequence of avoiding the typical Hartree-Fock error, obviating the distinction between exchange and correlation that is so ingrained in quantum chemistry after decades of the reigning paradigm: first Hartree-Fock, and then beyond.

We will argue that, once one is prepared to accept the computational complications and the price of dealing with orbital dependent functionals, it is still not necessary, and maybe even against the origin of the success of DFT, to go back to the exact-exchange starting point. It is possible to go back to the statistical definition of correlation in terms of two-electron probability distributions, or exchange and correlation holes,

1. M. A. Buijse and E. J. Baerends, Mol. Phys. 100 (2002) 401.

2. E. J. Baerends, Phys. Rev. Lett. 87 (2001) 133004.

3. M. Grüning, O. V. Gritsenko and E. J. Baerends, J. Chem. Phys. 118 (2003) 7183.

4. O. Gritsenko, K. Pernal, E. J. Baerends, J. Chem. Phys., to be published.