1.253(2) 0.000(0) 0.948(5) 0.948( 0.000(0) 0.002(6) 0.000(0) 0.000(0) 0.000(0) 0.000(0) 0.000(0) 0.000(0) RMSD -0.627 0.161 0.161 0.102 0.099 0.094 0.091 of all computed singlet states with respect to the reference, whereas the FEM yielded wrong results for S 6 -S 8 . This difference can be explained by examining the characters of the FEM and ECIS+FDA states. According to the ECI vectors (Tables and), FEM and ECIS+FDA agree on the description of the nearly degenerate S 1 -S 4 system states of G 4 as four linear combinations of the 1 ππ * 1 local excitations of the four guanines. In contrast, the S 5 -S 8 states differ between the methods-the FEM predicts four superpositions of 1 nπ * 1 local excitations, whereas ECIS+FDA predicts superpositions of 1 ππ * 2 local excitations. This difference has a similar explanation as the changing order of the site states with different APCs in G 4 (see Figure ). In particular, the 1 nπ * 1 excitation makes the O atom a worse hydrogen bond acceptor, so the excitation energy of 1 nπ * 1 states should rise in the hydrogen-bonded full system. Indeed, ECIS+FDA captures this effect through the diagonal cross-fragment interaction terms in Equation (8). The FEM neglects those interaction terms and thus misses the change in hydrogen bond strength. The effect of neglecting the interaction terms in FEM can most easily be seen by comparing the site state energies (Table )-4.423 eV for T 1 , 4.526 eV for T 2 , 6.359 eV for S 1 , and 6.857 eV for S 2 -with the FEM energies (Table ). The close agreement (within few meV) shows that for G 4 , FEM predicts excitation energies dominated by the site state energies.