To have direct full-system and ECI calculations on the same footing, we had to use the same level of theory in the site-state generation for ECI as in direct full-system calculation. In particular, we used CIS/def2-SVP, due to its affordability and ease of interpretation. Also, because it is variational, there is no need for additional modifications of the ECI expressions. We stress that in this case (using CIS as the level of theory in the site-state calculations), each LE ECSF is a linear combination of singly-excited full-system CSFs. Similarly, each DLE ECSF is a linear combination of doubly-excited CSFs having two excited MOs belonging to the two different fragments. Hence, an ECISD calculation might in principle produce some qualitatively correct fullsystem states that are not accessible by a direct full-system CIS calculation. In fact, in some of the ECISD calculations on several geometries of the G 4 and MgG 4 systems (presented below), we obtained several states with dominant DLE character. To assess whether those states are realistic, we have conducted a full-system CISD/SVP calculation on one of these geometries of MgG 2+ 4 , using the RASCI feature of the BAGEL package, employing small RAS1 and RAS3 (and empty RAS2) spaces and allowing up to two RAS1→RAS3 excitations. The included MOs were one n N7 orbital, the HOMO, the LUMO+1, and the LUMO+2 for each of the four guanines (orbitals are shown in Figure ; see Section 5 for the explanation of the chosen active spaces). In addition, one more occupied MO was included because it also contained a portion of n N7 character.