At the CC2 level, the first excited state of AZO-1 is NSCD dark at all nuclei, except for C2 and C7-that is, the nuclei connected to the azo group. This is explained by the fact that ES1 is an n → π * excitation with the hole orbital localized at the azo group, whose transition dipole moment (entering the B K expression) is only non-zero in the direction perpendicular to the molecular plane. At C2, B C is positive, whereas it is negative at C7. We attribute the non-negligible NSCD at C2 and C7 to the fact that ES1 is strongly localized on the azo group, see Figure . the NSCD signal "leaked" outside of the regions of high difference densities up to a distance of few bonds, which in the present case represents a significant fraction of the total molecule.