Exact density functional obtained via the Levy constrained search

A stochastic minimization method for a real-space wavefunction, $\Psi({\bf r}_{1},{\bf r}_{2}\ldots{\bf r}_{n})$, constrained to a chosen density, $\rho({\bf r})$, is developed. It enables the explicit calculation of the Levy constrained search $F[\rho]=\min_{\Psi\rightarrow\rho}\langle\Psi|\hat{T}+\hat{V}_{ee}|\Psi\rangle$ (Proc. Natl. Acad. Sci. 76 6062 (1979)), that gives the exact functional of density functional theory. This general method is illustrated in the evaluation of $F[\rho]$ for two-electron densities in one dimension with a soft-Coulomb interaction. Additionally, procedures are given to determine the first and second functional derivatives, $\frac{\delta F}{\delta\rho({\bf r})}$ and $\frac{\delta^{2}F}{\delta\rho({\bf r})\delta\rho({\bf r}')}$. For a chosen external potential, $v({\bf r})$, the functional and its derivatives are used in minimizations only over densities to give the exact energy, $E_{v}$ without needing to solve the Schr\"odinger equation.