Upon diagonalizing, the (M -O) eigenvectors with largest occupation numbers span the valence antibonding orbital space, and, together with the occupied space, complete the span of a molecule-adapted minimal basis. Localization of these valence virtual orbitals will then yield an alternative to the localized virtuals above. The advantage of this approach is for cases where there is no simple 1:1 mapping between bonding and antibonding orbitals, as discussed more later.