We do note that the orbitals associated with key amplitudes for strong correlation are not necessarily spatially localized. One method used to obtain initial guess antibonding orbitals is the so-called Sano procedure. In brief, after localizing a set of occupied orbitals using standard methods, the Sano procedure finds the virtual orbital that has maximum exchange interaction with each given localized occupied orbital. The idea of maximizing exchange is very old and comes from its predecessor, the modified virtual orbitals (note that modified virtual orbitals have been since used to refer to any non-canonical set of virtual orbitals ). The resulting orbitals are symmetrically orthogonalized to yield a set of valence antibonding orbitals.