For a hamiltonian H that describes the studied system and any normalizable function Ψ with arguments appropriate for the unknown wave function of the system, we define the functional ε ε [ Ψ Ψ ] = ⟨ Ψ Ψ | H ^ ^ | Ψ Ψ ⟩ ⟨ Ψ Ψ | Ψ Ψ ⟩ .