, where E is the excitation energy of an electronic transition for the aggregate, c n are components of the eigenvector of Ĥ, and µ mon is the transition dipole moment of the monomer. Since c n does not depend on separation, † the only difference in the oscillator strength comes from E, which, in turn, is larger for smaller d. We also note that PDM oscillator strengths are larger than those calculated by first-principles methods (Fig. , lower row). Further, for the decamer d = 3.5 Å, we calculated absorption spectra using the methods applied to the monomer. The spectra are shown in Fig. . The position of the ππ * band follows the same trend (with respect to the used methods) as for the monomer. We note that the B3LYP curve (the black line y ≈ 0 in the upper panel) does not show the intense ππ * peak, since the requested 50 lowest excited states turn out to be not enough to reach it (the TD-B3LYP 50th excited state is located at 3.13 eV, which is below the S 0 → S 2 transition of the monomer at 3.85 eV, compare with Fig. ). The brightest state labels and energies calculated with different methods as well as the blue shifts with respect to the monomeric S 0 → S 2 transition are listed in Tab. 2. The semiempirical methods and TD-HF predict the brightest transition to be the S 0 → S 20 transition, agreeing with the simple exciton model. Ab initio CIS and TD-DFT, on contrary, predict the brightest state label to be larger than 20, thus placing some additional states (which are not expected in the simple exciton model) below it. In particular, CIS yields S 0 → S 25 as the most intense transition, whereas the TD-DFT brightest state labels vary from 28 to 44, depending on a functional. In particular, for the used TD-DFT methods, the more blue-shifted is the ππ * absorption band the smaller is the brightest state label. For larger intermolecular distances, however, TD-DFT labels become smaller, e.g. 20 for ωB97X-D, for the decamer with d = 5.5 Å. Further, we analyzed the charge transfer contribution to the calculated excited states for TD-ωB97X-D and CIS spectra, for decamers d = 3.5 Å and d = 5.5 Å. The sum of off-diagonal (CT) elements of FTDM matrices for the lowest 50 excited states of these decamers is shown in Fig. . The brightest transitions are marked with stars there. It is seen that for d = 5.5 Å, the CT contribution is virtually zero for transitions below the brightest one. In fact, it is zero for all 50 calculated transitions at the CIS level, whereas for ωB97X-D the first 30 transitions are charge transfer free and the next 20 transitions are entirely of the CT type. For d = 3.5 Å, on contrary, the observed picture is more complex. First ten transitions (the nπ * transitions) show (very) minor CT, which is more pronounced for ωB97X-D than for CIS. For the higher energy transitions the CT contribution becomes larger, e.g. 37 % (ωB97X-D) or 21 % (CIS) for the S 0 → S 11 transition. And some of the transitions with even higher excitation energies are the CT-dominated transitions. For the brightest transitions the CT contribution is small (∼8 % for ωB97X-D and ∼2 % for CIS).