It is customary to expand the aggregate's eigenstates on the basis of so-called n-particle excitation states. In what follows we will consider a molecular dimer and denote the monomer ground state by g m and any of the excited states by e m (Fig. ). Hence the zero-particle state is defined as |g m g n ⟩ and there are two one-particle states, i.e., |e m g n ⟩ and |g m e n ⟩. While zero-and one-particle excitations are sufficient to treat processes related to linear absorption and exciton dynamics, two-particle excitations, |e m e n ⟩, will be important for the treatment of nonlinear spectroscopies 18 . Thus, the eigenstates, |Ψ κ ⟩, of the FEH will be obtained according to the following expansion