The overall shape of the spectrum for the large model system is largely preserve compared to the spectra for the smaller model systems. There is a small increase in the intensity of the positive band as compared to the medium model system, but this effect is barely significant. More important, however, is the qualitative feature change in the long-wavelength region with respect to the small and medium models. There is here the appearance of a negative band around 250 nm that is also found present in the experiment but which has so far not been reproduced in theoretical work-we also did not find any experimental attribution for this band. In CPP calculations of ∆ϵ(ω), we can decompose the final result into contributions from individual orbital excitations. In general, such an analysis will show that for any value of the angular frequency, there are a large number of excitations that contribute significantly with varying positive and negative values. In the region of the negative band, however, an unexpected observation can be made namely the involvement of moieties other than the guanines. In Fig. , we illustrate two excitations that both contribute with negative values to the Cotton effect. First, the excitation from occupied orbital HOMO-249 to unoccupied orbital LUMO+29, illustrated in the lower figure panel, is shown to be influenced by the potassium ions and which suggests the possible participation of the stabilizing cation in the formation of the CD spectrum. Second, the excitation from occupied orbital HOMO-547 to unoccupied orbital LUMO+7, both of which are localized on the thymine and adenine nucleobases in the link sequence (upper figure panel). This attribution of the TTA linkers as contributors to the negative band in the long-wavelength region falls in line with an earlier study that suggested that the excitation energy of thymine is lower than that of guanine.