As mentioned above, the excitonic basis in the FEM contains only GS and LE products, which can be written in the present notation as |0⟩ = |10, . . . , M 0⟩ and 0 F a F F 0 = |10, . . . , F a F , . . . , M 0⟩, respectively. Just like in the previous section, 0 F a F F 0 denotes that the LE product is obtained from the GS product by exciting the site F from site state F 0 to F a F . In the following, we will occasionally drop out the symbol of the site ground state in the subscript (i.e., we write 0 F a F instead of 0 F a F F 0 ) to simplify the notation. The excitonic basis in the FEM is not complete, as the complete excitonic basis should also contain all MLE products, and additionally charge-transfer (CT) products. The latter are products which include cationic and anionic site states (relative to some reference charge distribution per sites), but having the same total charge as the GS, LE, and MLE products.