  In this paper we propose the idea of expanding the space of variations in
standard variational calculations for the energy by considering the wave
function $\psi$ to be a functional of a set of functions $\chi: \psi =
\psi[\chi]$, rather than a function. In this manner a greater flexibility to
the structure of the wave function is achieved. A constrained search in a
subspace over all functions $\chi$ such that the wave function functional
$\psi[\chi]$ satisfies a constraint such as normalization or the Fermi-Coulomb
hole charge sum rule, or the requirement that it lead to a physical observable
such as the density, diamagnetic susceptibility, etc. is then performed. A
rigorous upper bound to the energy is subsequently obtained by variational
minimization with respect to the parameters in the approximate wave function
functional. Hence, the terminology, the constrained-search variational method.
The \emph{rigorous} construction of such a constrained-search--variational wave
function functional is demonstrated by example of the ground state of the
Helium atom.
