Therefore, the nπ * absorption band, located between 2.0 and 3.3 eV (Fig. , left) is of a particular interest here. is forbidden for the optimized trans ground-state geometry processing the C 2h symmetry. However, the transition acquires some intensity via coupling to vibrations and is well observed in experiments. Since the transition dipole moment is zero at the equilibrium geometry, it is not possible to predict the changes in excitation energy and oscillator strength upon aggregation using the molecular exciton theory 2 for the nπ * transition (assuming identical, C 2h geometries of the monomers).