The density functional theory (DFT) treats the electron density as the central variable rather than the many-body wavefunction. This conceptual difference leads to a remarkable reduction in difficulty: the density is a function of three variables, *i.e.* the three Cartesian directions, rather than variables as the full many-body wavefunction is. An early density functional theory was proposed by Thomas and Fermi [45]. This took the kinetic energy to be a functional of the electron density, but in common with the Hartree and Hartree-Fock methods, only incorporated electron-electron interactions via a mean field potential: as such it neglected *both* exchange and correlation; a subsequent proposal by Dirac [46], formulating an expression for the exchange energy in terms of the electron density failed to significantly improve the method. Here we consider the Hohenberg-Kohn-Sham formulation of DFT; this technique is one of the choice state-of-the-art methods routinely applied in electronic structure theory, and has enjoyed success in fields ranging from quantum chemistry and condensed matter physics to geophysics. It is based upon the following remarkable and deceptively simple theorems:

- Theorem 1.
*The external potential is a unique functional of the electron density only. Thus the Hamiltonian, and hence all ground state properties, are determined solely by the electron density.*

The many-body Hamiltonian *H* fixes the groundstate of the system under consideration, *i.e.* it determines the groundstate many-body wavefunction , and thus the above theorem ensures that this itself is also a unique functional of the groundstate density. Consequently, the kinetic and electron-electron interaction energies will also be functionals of . One may therefore define the functional

(2.10) |

Using this functional, one may then define, for a given external potential , the energy functional

(2.12) |

(2.13) |

(2.14) |

*Proof:*We proceed by*reductio ad absurdum*. Assume that there are two potentials and that differ by more than an additive constant and further that these two potentials lead to different ground state wavefunctions and . Now assume that these both lead to the same ground state density, . The variational principle then asserts that

(2.15)

Note that this inequality applies only to the groundstate and that DFT, as a result, is only rigorously applicable to the groundstate. Interchanging 1 and 2 gives a similar expression, and adding the two inequalities leads to the contradiction

(2.16) Thus theorem 1 is proved.

- Theorem 2.
*The groundstate energy may be obtained variationally: the density that minimises the total energy is the exact groundstate density.*

*Proof*. To prove this theorem we introduce the notion of ``*N*-representability'':*i.e.*a density is said to be*N*-representable if it may be obtained from some antisymmetric wavefunction for which we may define the functional [47]

where the minimum is taken over all that yield the density

*n*. Now, if we introduce for a wavefunction that minimises 2.17 such that

(2.18) then

(2.19)

with equality at the minimum. Thus the second theorem is proved.

Although these two theorems prove the existence of a universal functional, they do not give any idea as to the nature of the functional, or how to actually calculate the groundstate density. In order to do so, we must discuss the Kohn-Sham formulation [38]. This is based upon a sleight of hand whereby we map the fully interacting system of *N*-electrons onto a fictitious auxiliary system of *N* non-interacting electrons moving within an effective Kohn-Sham potential,
, thereby coupling the electrons. The single-particle Kohn-Sham orbitals are constrained to yield the same groundstate density as that of the fully-interacting system, so the Hohenberg-Kohn-Sham theorems are still valid.

Variation of the total energy functional in 2.11 with respect to the electron density, subject to the constraint of fixed particle number, *i.e.*

yields

where is a Lagrange multiplier associated with our constraint condition 2.20. The Euler-Lagrange equation associated with minimisation of this functional is then

(2.22) |

The Kohn-Sham formulation allows us to write the universal functional as

(2.23) |

where the last term is the *exchange-correlation* energy, to which we will return presently, and
is the kinetic energy, which may be written in terms of the *non-interacting* single-particle orbitals as

(2.24) |

It is important to note that this is the kinetic energy of the auxiliary non-interacting system, *not* the kinetic energy of the actual physical system under consideration.
is the classical Hartree energy of the electrons

(2.25) |

Thus the Euler-Lagrange equation 2.21 becomes

(2.26) |

where is the effective Kohn-Sham potential

(2.27) |

The Hartree potential is given by

(2.28) |

with the exchange-correlation potential

(2.29) |

The Euler-Lagrange equation is now of exactly the same form as that which leads to the Hartree equations 2.4. Therefore we are required to solve the Schrödinger-type equations

where the correspond to the eigenvalues of the single-particle states and the charge density is constructed from the Kohn-Sham orbitals as

(2.31) |

Similarly, the many-electron wavefunction of the system may be constructed as a Slater determinant of the Kohn-Sham orbitals.

The Kohn-Sham formulation thus succeeds in transforming the -body problem into single-body problems, each coupled via the Kohn-Sham effective potential. It is worth noting that formally there is no physical interpretation of these single-particle Kohn-Sham eigenvalues and orbitals: they are merely mathematical artefacts that facilitate the determination of the true groundstate density. The exception is the highest occupied state, for which it can be shown that [47] the eigenvalue corresponding to the highest occupied state yields the ionisation energy of the system.