We present a general method for extracting the internal space of an arbitrary wave function, exploiting the molecular symmetry. This permits the rewriting of a wave function built with nonorthogonal orbitals as a compact expansion of Slater determinants over orthogonal orbitals. The method is applied to valence bond (VB) and projected unrestricted Hartree-Fock (PUHF) functions along the potential curves of some diatomics, namely He-2(+), F-2(-), and F-2. These functions are then used as references for further configuration interaction and perturbative Moller-Plesset calculations.