Although formally involving fewer approximations than the FEM, the ECIS+FDA method shows a significantly worse performance on MgG 4 . We find RMSDs in the energies of 0.6 eV for the triplets and 0.8 eV for the singlets, originating from a systematic overestimation of all excitation energies. The reason for this sys-tematic blueshift is the inclusion of the GS-LE couplings, which are non-zero due to the FDA, because the site states are far away from selfconsistency. In this system, these couplings are much larger than in G 4 , because Mg 2+ strongly couples the GS product and any LE product through the interaction of the ground-state density (and the nucleus) of Mg 2+ with the transition density on the excited guanine molecules. This coupling is described by a term in Equation (9) corresponding to G = 5. The non-zero GS-LE couplings in MgG 2+ 4 lead to the loss of size-extensivity, which in turn leads to an artificial increase in the splitting between the full-system ground state and the excited states, resulting in the aforementioned blue shift. In contrast, the FEM neglects the GS-LE couplings from the outset, and therefore is in principle size-extensive. Nevertheless, the S 1 -S 4 states on ECIS+FDA level seem to be qualitatively correct, with the S 1 state having a small non-zero oscillator strength, the degenerate pair S 2 -S 3 being very bright, and S 4 state being dark. On the other hand, according to the oscillator strengths, S 5 -S 8 states are not qualitatively correct.