The difference in monomer HOMO energies and the difference in monomer LUMO energies are defined as "Δ 2343 " and "Δ 5643 " respectively. The LUMO en ergy of a co-dimer will experience a greater contribution from the monomer with the lower lying LUMO orbital. Conversely, the dimer HOMO energy will experience a greater contribution from the monomer with the higher lying HOMO orbital. The difference between these molecular orbital energies is defined as the "Dimer shift". An illustration of these definitions is shown in Figure .