, | (6.1) |

where is the one-electron (kinetic plus potential) energy, is the classical Coulomb repulsion of the electrons, is the exchange energy resulting from the quantum (fermion) nature of electrons, and is the nuclear repulsion energy.

In density functional theory, the exact Hartree-Fock exchange for a single determinant is replaced by a more general expression, the exchange-correlation functional, which can include terms accounting for both exchange energy and the electron correlation which is omitted from Hartree-Fock theory. The DFT energy is expressed as a functional of the molecular electron density ,

, | (6.2) |

where is the kinetic energy, is the nuclei-electron interaction, and are the exchange and correlation energy functionals.

The exchange and correlation functionals normally used in DFT are
integrals of some function of the density and possibly the density
gradient. In addition to pure DFT methods, `dscf` and `grad` modules
support hybrid functionals in which the exchange functional includes
the Hartree-Fock exchange, e.g. B3-LYP.