Density Functional Theory (DFT) is based on the notion of the single particle electron density as a fundamental variable. This is a consequence of the Hohenberg - Kohn theorem [11] which states that the ground state electron density minimises the energy functional

where is a universal functional. Furthermore, the minimum
value of *E* is , the ground state electronic energy. This is an
exact result and in principle, means that the ground state energy and
electron density may be found using a variational minimisation over
the electron density , a process with scales linearly with
system size.

Unfortunately, the proof of the Hohenberg - Kohn theorem is not constructive, hence the form of the functional in Equation is not known. Kohn and Sham postulated [12] that this functional could be written

Here the first term is the kinetic energy of a system of non-interacting electrons with density and the second is the electron - electron Hartree interaction. The final term, , is the exchange-correlation energy. By writing the electron density in terms of a set of single particle wavefunctions such that

the kinetic energy term may be written

Minimising with respect to , subject to the constraint that the number of electrons must be constant, leads us to a set of equations

where is an `effective potential',

and is the exchange-correlation potential . Equation can be seen to be a set of Schrödinger-like equations for the single particle Kohn-Sham orbitals

Thus, the problem of a system of interacting electrons has been
mapped onto a system of non-interacting electrons moving in an
effective potential given by Equation . However, the expression of
the density in terms of a set of single particle orbitals has
increased the complexity of the problem. The minimisation must now be
performed over *NM* degrees of freedom where *M* is the number of
basis functions used to represent the .

Wed Sep 24 12:24:18 BST 1997