However, whether one encounters weak or strong excitonic correlation does not only depend on how strongly the fragments are interacting, but also how the site states were constructed. In particular, excitonic correlation in ECI depends on the specific choice of the APCs, just like electronic correlation in (uni)molecular systems depends on the specific set of MOs used in the CI calculation. It should be stressed that, in our approach, excitonic correlation is also of relevance for the ECI ground state, which might exhibit some contributions of LE and MLE products (through GS-LE and GS-MLE couplings) in addition to the (likely dominating) GS product. Such excitonic correlation of the ground state is not possible in FEM and FEM-like models, as they neglect GS-LE couplings-in these models, the ground state wave function is always equal to the GS product.