While the Coulson-Rushbrooke orbital pairing theorem is generally applied to the π-system of conjugated molecules, the mathematical form of the ladder C model we employ means that it also applies to butane. In this case however, it is very clear that the interference is not between the paired orbitals (shown as the rows in Figure :HOMO-2 and LUMO+2, HOMO-1 and LUMO+1, HOMO and LUMO) as the phase does not differ. Instead, from Figure , Figure and Figure it is clear that the balance between constructive and destructive interference is only shifted by the varying magnitude of the destructive interference between the pair of HOMO-1 and LUMO+1 with primarily the HOMO and LUMO (and to a lesser extent the HOMO-2 and LUMO+2). The orbital pairing theorem means that the energetic position of paired orbitals will always be symmetrical above/below 0eV as well as that the widths of the transmission peaks are identical. Together this means that when interference is between paired orbitals, the interference dip must also be at 0eV. On the other hand, when the interference is between non-paired orbitals, the details of the balance between orbital positions and weights mean that the interference features can shift in energy as well as appearing/disappearing, as seen in Figure (c).