For questions about simulations using Density Functional Theory.
Density Functional Theory is an electronic structure theory model developed by Pierre Hohenberg and Walter Kohn in the early 1960s. The so called Hohenberg-Kohn theorems show that (1) The ground state energy of an N-electron system is uniquely determined by its electron density (2) There exists a functional of the ground state energy and the exact ground state electron density minimizes this functional.
These theorems in principle allow the complicated problem of determining the 3N dimensional wavefunction of the system via the Schrodinger equation to be reduced to the problem of determining the 3 dimensional electron density that minimizes the energy functional.
Kohn-Sham (KS) DFT refers to a version of DFT in which the electrons are treated as noninteracting and moving within an effective potential.
The exact energy functional (in particular, the exchange-correlation portion of the KS potential) is not expressible analytically, so numerous approximate functionals have emerged