For MgG , going from ECIS+EHF to ECISD+EHF does not further improve the results, as the site states are of such quality that the excitonic singles are sufficient to correctly describe the dominantly LE full-system states. Although ECI+EHF produces decent results in MgG 4 , we note that the obtained RMSDs are still larger than in G 4 . Given that the set of site states with EHF embedding and ECI expansion are converged, this residual error in the spectrum can only be prescribed to a certain degree of SOA violation, which in MgG 4 system comes from the proximity of the Mg 2+ cation and the guanines.