Frequently, one may refer to a "donor" or "acceptor" monomer, in regards to an electron donor with high HOMO orbital energy and an electron acceptor with low LUMO orbital energy. Unfortunately, since the HOMO-LUMO gaps of the two monomers may be different, the real picture is more complex. Figure illustrates four common orbital mixing motifs that repeatedly occur in this data set. In cases (a) and (c), one monomer is both the "HOMO donor" and "LUMO donor", while the other is both the "HOMO acceptor" and "LUMO acceptor". In cases (b) and (d), differences in Eg energies lead to one monomer acting as the "HOMO donor" and "LUMO acceptor" while the other acts as the "HOMO acceptor" and "LUMO donor". Case (a) exemplifies ideal behavior with respect to Hückel molecular orbital (HMO) theory. After combining the two monomers, their electron densities become delocalized across the conjugated dimer. This leads to an increase in the dimer HOMO energy, a decrease in the dimer LUMO energy and a decrease in the overall Eg energy. In case (b), large difference in monomer Eg energies lead to polar, localized dimer molecular orbitals. Cases (c) and (d) display unexpected (but common) orbital mixing behaviors. In case (c), the dimer molecular orbitals exhibit a mixing pattern that is characteristic of neither electron delocalization nor localization. Large dipole moments lead to charge transfer interactions that dominate the observed dimer molecular orbital energies. For example, the calculated dipole moment of the dimer in case (c) is 4.9 Debye, compared to 1.7 Debye for the dimer in case (a). In case (d), the dimer molecular orbitals are localized, despite forming a conjugated aromatic system.