In computational condensed-matter physics, the Harris energy functional is a non-self-consistent approximation to Kohn-Sham density functional theory

Density functional theory

Density functional theory is a quantum mechanical modelling method used in physics and chemistry to investigate the electronic structure of many-body systems, in particular atoms, molecules, and the condensed phases. With this theory, the properties of a many-electron system can be determined by...

. It gives the energy of a combined system as a function of the electronic densities of the isolated parts.
The energy of the Harris functional varies much less than the energy of the Kohn-Sham functional as the density
moves away from the converged density.
Therefore, for many systems the accuracy without the self-consistency may be sufficient. The Harris functional was
originally developed for such calculations rather than self-consistent convergence, although it can be applied in a
self-consistent manner in which the density is changed.
While the Kohn-Sham DFT energy is Variational method (never lower than the ground state energy), the Harris DFT energy was originally believed to be anti-variational (never higher than the ground state energy). This was however conclusively demonstrated to be incorrect.
Harris functional is used in
some codes, such as Fireball.