# 8.7: Density functional theory (Nobel prize 1998)

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If we consider the total probability density of a system of many interacting particles $$\rho ({\bf r})$$, there may be several possible wavefunctions which could give rise to it: call this set $$S(\Phi)$$.

Now, consider the expectation value of the energy $$\langle \hat{H} \rangle$$. We know from the variational principle that $$\langle \hat{H} \rangle \geq E_o$$. If we define a functional $$F[\rho ({\bf r})] = \text{Min}_{S(\Phi )} \langle \hat{H} \rangle$$, then it follows that $$F[\rho ] \geq E_o$$.

Consequently we can use the variational principle to find the $$\rho ({\bf r})$$ which minimises the value of F, and this may give us the ground state energy without having to evaluate the wavefunction. This is especially useful when the wavefunction consists of complex combinations of many different single-particle wavefunctions, as with the many electrons in a solid or molecule.

The drawback is that for interacting electrons, the functional is not known.

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