Second, potential reconstruction schemes can be used to study fundamental aspects of sDFT/FDE. Because of a claim in the literature suggesting that sDFT can formally only be exact if the orbitals of different subsystems are mutually orthogonal , which was picked up also in later work , our groups used potential reconstruction techniques to obtain accurate reference potentials for molecular sDFT calculations with finite basis sets . Together with analytical examples and basis-set-free numerically calculations, these computations proved that there is no need for requiring this so-called external orthogonality in sDFT calculations. Nevertheless, external orthogonality (EO) -which is effectively equivalent to projector-based embedding (see Section 6.1) -can be used in density-based embedding as a workaround, i.e., instead of non-additive kinetic-energy approximations.