The ECI formulas work for any (arbitrary) choice of APCs. Therefore, the APCs can be viewed as tuning parameters that only modify the site states (and hence the ECI basis) without modifying the full-system Hamiltonian operator (as it does not depend on APCs, see Equation ( )). In particular, we stress that the APCs in ECI are not used to compute the interfragment interaction energy within the diagonal matrix elements (unlike in the abovementioned exciton models ), as the ECI formulas already contain terms taking into account all contributions to matrix H. Nevertheless, due to their influence on the site states, APCs are expected to still strongly influence the overall performance of ECI. This is analogous to the effect of the choice of the MOs to the performance of a CI calculation. Consequently, the question arises what is the best choice of APCs. One possible choice, following the logic of the FEM, is to set the APCs of all atoms in the system to zero, which results in ECI with FDA. In this case, the modified ECI formulas (Equation ( )) directly simplify to the original GFT formulas (Equations ( ), ( ), (10)). Another choice is to use partial atomic charges that reproduce the electrostatic potential of the individual chromophores in their ground states (so-called ESP charges). These can be obtain by various regression procedures applied on the ground-state density. This choice would result in ECI with similar embedding employed in the described exciton models employing QM-MM electrostatic-embedding scheme, since the MM-FF point charges are usually constructed as the ESP charges of the molecular ground state. However, in an analogy with McWeeny's variational calculation for a single antisymmetrized product (described at the end of the Section 2.1), a better choice should be the set of APCs that minimize the full-system ground-state energy (E 0 ), using the APC-embedded GS product