The performance of the ECI method for the calculation of electronic absorption spectra is illustrated with the two multichromophoric systems shown in Figure . The first test system is a single layer of the guanine-quadruplex (G 4 ), exhibiting double hydrogen bonds between guanine moieties connected in the supramolecular ring. This motif is often found in the telomeric regions of DNA and is often stabilised by a metal cation. Hence, the second test system is G 4 with an Mg 2+ cation in the center (MgG ), which constitutes the more challenging test due to thepresence of metal-ligand bonds and of stronger cross-fragment polarization. The G 4 system was treated as a four-fragment system (each guanine as a single fragment) and the MgG 4 system as a five-fragment system, with Mg 2+ cation being one fragment for itself (computational details in Section 4). The performance of ECI on these two systems will be presented and discussed in Section 5.