A more accurate description of the ECD spectrum should include vibronic effects. These contributions can come from modes localized on a single molecular unit within the extended system, through interactions between neighbors, or as collective effects of the extended system. The first effect can be treated with techniques used for molecules, see again figure ??. Models for interactions between molecular units have also been developed within an excitonic picture. The last type of interaction, exciton-phonon coupling, is harder to describe and it requires a vibrational calculation including PBCs.