Determination of a Wave Function Functional

We propose expanding the space of variations in traditional variational calculations for the energy by considering the wave function ψ to be a functional of a set of functions χ:ψ=ψ[χ], rather than a function. A constrained search in a subspace over all functions χ such that the functional ψ[χ] satisfies a sum rule or leads to a physical observable is then performed. An upper bound to the energy is subsequently obtained by variational minimization. The rigorous construction of such a constrained-search--variational wave function functional is demonstrated.