Editing Quantum chemistry (section)

=== Density functional theory ===
{{main|Density functional theory}}

The [[Gas in a box|Thomas–Fermi model]] was developed independently by [[L. H. Thomas|Thomas]] and [[Enrico Fermi|Fermi]] in 1927. This was the first attempt to describe many-electron systems on the basis of [[electronic density]] instead of [[wave function]]s, although it was not very successful in the treatment of entire molecules. The method did provide the basis for what is now known as density functional theory (DFT). Modern day DFT uses the [[Kohn–Sham equations|Kohn–Sham method]], where the density functional is split into four terms; the Kohn–Sham kinetic energy, an external potential, exchange and correlation energies. A large part of the focus on developing DFT is on improving the exchange and correlation terms. Though this method is less developed than post Hartree–Fock methods, its significantly lower computational requirements (scaling typically no worse than ''n''<sup>3</sup> with respect to ''n'' basis functions, for the pure functionals) allow it to tackle larger [[polyatomic molecule]]s and even [[macromolecule]]s. This computational affordability and often comparable accuracy to [[Møller–Plesset perturbation theory|MP2]] and [[Coupled cluster|CCSD(T)]] (post-Hartree–Fock methods) has made it one of the most popular methods in [[computational chemistry]].