Throughout the work, the threshold t O from equation ( ) was set to 0.95, while the threshold t Q in EHF was set to 10 -4 e. In all ECI calculations, the set of site states on each guanine moiety comprised S 0 -S 7 and T 1 -T 3 . For the Mg 2+ fragment in MgG 2+ 4 , we included only the S 0 site state, because the excited site states occur at excitation energies higher than 50 eV and hence are not expected to influence the spectrum in the low-energy region in which the local excitations of the guanines are found. The employed geometries of G 4 and MgG 2+ 4 were optimized in the ground state, using a full-system HF calculation, without the use of symmetry. The optimized geometries of G 4 and MgG 2+ 4 were found to belong to the C 4h and S 4 point groups, respectively, and were subsequently reoptimized in the largest subgroups available in Gaussian (C 2h and C 2 , respectively). Hence, all four guanine molecules are symmetry equivalent in both systems, and therefore they will all have the same site states. However, exact degeneracy was only found with FDA, while with ESP and EHF embedding the site states varied slightly because the ESP charges obtained by Gaussian slightly depend on the molecular orientation. The excitation energies of the first eight singlet and first eight triplet excited states of the full system were compared. For the excited singlet states, transition dipole moments and oscillator strengths from S 0 were also calculated.