The ECI wave functions of those states (Table ) nicely display the high excitonic correlation in this system when the FDA is employed. The S 0 exhibits about 5% admixture of LE products (E 1 diagnostic in the table) due to the non-Brillouin GS-LE couplings. The states S 1 -S 3 are superpositions of four S 1 local excitations and four S 3 local excitations. This is quantified by the participation ratio PR in Table , which shows that S 1 is dominated by four equally contributing excitations, and S 2 /S 3 by two such excitations. In contrast, the S 4 state has significant coefficients for the GS product and the S 1 and S 3 local excitations, and some contributions from higher states. Consequently, the PR for S 4 is much higher (about 7). The second set of singlet states (S 5 -S 8 ) exhibits an even higher degree of mixing, with PR values of about 6 to 11. This also suggests that, in addition to the non-adequateness of the ECIS expansion and the FDA embedding, the used set of site states might not be sufficiently converged (i.e., that too few site states are supplied per fragment) for the description of the higher 0.000(0) 0.000(0) 0.000(0) 0.000(0) 0.000(0) 0.000(0) 0.000(0) 0.000(0) S 5 0.000(0) 0.000(0) 0.095(0) 0.087(2) 0.788 (6) system. In general, the strong mixing between GS and LE products, as well as between two same-site LE products, is again a consequence of the strong interactions between each guanine and Mg 2+ , giving rise to large J terms in Equation .