New forms of complexity emerge in systems having multiple electronic chromophores that are identical or near-identical and whose vertical excitation energies are therefore quasi-degenerate. If the electronic coupling between chromophores is sufficiently strong, then the monomer excitations will mix and the excited-state wave function for the aggregate system will be delocalized across more than one chromophore. Consider the case of two identical monomers in a high-symmetry arrangement, such as a cofacial benzene dimer with D 6h symmetry, for which the pNTOs are illustrated in Fig. . In Section 3.4, it was discussed that a minimum of four Slater determinants is required to describe the frontier excitations of the benzene monomer [Eq. (3.7)], and the same is true for the dimer but the relevant pNTOs are delocalized over both monomers.