Finding a suitable set of antibonding orbitals provides the missing part of the valence space not spanned by the occupied orbitals. Thus the union of the occupied space and the space of antibonding orbitals spans the space of an effective minimal basis. It is well accepted that full valence CASSCF wavefunction is spanned by an effective minimal basis within the molecule for this reason. Accordingly, localizing the union of the occupied orbitals with the antibonding orbitals reveals a set of molecule-adapted atomic orbitals (MAOs):