Testing the ECI method on the cases of G 4 and MgG 4 systems leads to a number of conclusions. For a supramolecular (e.g., hydrogenbonded) multichromophoric system, like G 4 , ECIS+FDA can be expected to yield qualitatively correct electronic states, with expected errors of excitation energies around 0.1-0.2 eV. For the same systems, ECISD+FDA might reduce the errors below 0.1 eV, giving both qualitatively and quantitatively correct full-system states. Using either ESP or EHF atomic charges as the APCs in such systems is expected to yield spectra in almost perfect agreement with ones obtained by direct quantum-chemistry method, already within ECIS.