Collective excitations of electronically coupled chromophores can be conceptualized as linear combinations of basis states |Ψ * 1 Ψ 2 and |Ψ 1 Ψ * 2 , in which one monomer or the other is excited. These are the Frenkel exciton (FE) states, as in the classic case of H-and J-aggregates of PAH molecules. In a high-symmetry system such as the benzene dimer, the mixing coefficients are equal: