We consider first the dodecamer 2D model with arrangement of molecules shown in Fig. . The spectra of the dodecamer calculated with various methods are shown in Fig. . Again, the order of the methods regarding the position of the most intense band is the same as for the monomer (compare with Fig. ), and this order agrees with that for the linear decamer (see Fig. ). The B3LYP "spectrum" again shows no bands (the black line y ≈ 0 in the upper panel) as in the case of the decamer, since 80 excited states requested in the present case are not enough to reveal the intense dodecamer ππ * transition with B3LYP (the 80th state is located at 3.16 eV). Vertical grey lines show the experimental, gas-phase monomer peak position. Comparing to the linear decamer, the monomer-to-aggregate blue shift (of the ππ * transition) is slightly larger, ∼0.4 eV for the 2D dodecamer vs. ∼0.3 eV for the 1D decamer at the TD-DFT levels used here (compare Tabs. 3 and 2). The dodecamer brightest state labels are higher than those for the decamer, e.g. 51 vs. 43 at the TD-ωB97X-D level. TD-HF and the used semiempirical methods (with exception of TD-AM1) again agree with the simple exciton model, predicting the brightest state to be 24 (see Tab. 3). TD-AM1 predicts it to be 36, but we should note that the TD-AM1 density of states is very large in the region of excitation energies close to the brightest state -TD-AM1 state 24 is at ∼3.66 eV and state 48 is at ∼3.70 eV. Furthermore, we computed the spectra of a larger model, octadecamer shown in Fig. . Additional blue shift and absorbance enhancement of the most intense peak are observed (Fig. ). Specifically, in comparison to the dodecamer the ππ * band is blue-shifted by ∼0.06 (0.07) eV and the maximum absorbance is enhanced in ∼1.3 (1.3) times at the TD-ωB97X-D (CIS) level. We also note some weak absorbance near and below the monomer S 0 → S 2 transition. The brightest state labels are 97 and 48 for TD-ωB97X-D and CIS calculations, respectively. The FTDM matrices for the transitions to these states are shown in Fig. , left column. The dominant contributions are on the diagonal. For ωB97X-D, there are also some nonzero CT contributions between π-stacked nearest neighbours (see molecule numbering in Fig. , right). The spatial distribution of local excitations (F XX elements) is shown in Fig. , middle column. It is seen that the brightest exciton exhibits the largest contribution from the central dimer, similarly to the case of the linear decamer (compare to Fig. ). Next, we calculated the absorption spectra of the SAM models shown in Fig. ,f, which are constructed using the experimental information on lattice parameters and orientation of azobenzenes with respect to a surface, as reported in Ref. 9. The spectra calculated with TD-ωB97X-D and CIS are shown in Fig. . The most intense band is more blue-shifted for the 18a model (Fig. ) than for the 18b model (Fig. ). Specifically, the 18b-to-18a shift is ∼0.10 eV for TD-ωB97X-D and ∼0.05 eV for CIS. Another remarkable difference is a more pronounced low-energy absorbance for the 18b model. In this respect we note that an appearance of the allowed, red-shifted transition is expected already for the unit cell dimer, since the two molecules of the unit cell are not parallel to each other. Indeed, we find for the isolated dimer, corresponding to the unit cell of 18b, the S 0 → S 3 transition at 3.89 eV with oscillator strength f = 0.20, in addition to the S 0 → S 4 transition at 4.17 eV with f = 1.05 (at the TD-ωB97X-D level). For the isolated dimer from 18a, on the contrary, the S 0 → S 3 transition at 4.08 eV is dark (f = 0.00), and only the S 0 → S 4 transition at 4.27 eV is bright (f = 1.49), see Fig. . Interestingly, for the dimer from model 18b, the S 3 and S 4 states are somewhat localized on either monomer (Fig. ), in contrast to the simple exciton model, in which each monomer contributes 50 % (c n = 1/ √ 2) to the overall excitation. This demonstrates the importance of specific molecular orientation at short distances. We also note that the localization of excitons on a single monomer was reported for edge-to-face benzene dimers. The brightest transitions for 18a are S 0 → S 44 (with ωB97X-D) and S 0 → S 36 (with CIS). For 18b, it is S 0 → S 36 with both methods. The FTDM matrices for the brightest transitions are shown in Fig. and 16 for 18a and 18b, respectively. For 18a (Fig. ), TD-ωB97X-D and CIS matrices look similar. The exciton is delocalized over the entire aggregate with the largest contribution from the two central molecules located on the spatial antidiagonal (Fig. , middle column). There are also minor CT contributions between molecules in close contact, e.g. 2 ↔ 3, 2 ↔ 7, etc. We also note that the TD-ωB97X-D FTDM matrix possesses some slight asymmetry, e.g. F 9,9 = 6.4 % ̸ = 6.3 % = F 10,10 . We relate it to the fact that S 44 is degenerate with S 43 at the TD-ωB97X-D level (both states are 4.3864 eV above the ground state; the corresponding oscillator strengths are 0.001 for S 43 and 3.903 for S 44 ). Previously we encountered such a situation for an azobenzene dimer. In the case of 18b, the difference between TD-ωB97X-D and CIS FTDM matrices is large (Fig. ). The CIS brightest exciton is more delocalized than the TD-ωB97X-D exciton. The largest diagonal element of the CIS FTDM matrix is F 10,10 = 8.4 %, whereas for TD-ωB97X-D it is F 11,11 = 21.4 %. We note, however, that the stick spectra obtained with the two methods are qualitatively different. While CIS predicts one strong transition (S 0 → S 36 at 5.03 eV with f = 8.11), TD-ωB97X-D predicts a series of closely lying transitions with smaller oscillator strengths (the most intese among them are S 0 → S 36 at 4.29 eV with f = 2.00 and S 0 → S 38 at 4.30 eV with f = 1.49), see Fig. . Finally, we performed a partial geometry optimization for an octamer model selected from 18a (see Fig. ), and calculated the spectra for the initial and optimized structures (Fig. ). The intense ππ * band for the optimized structure is blue-shifted by ∼0.06 eV in comparison to the band for the initial structure. In both cases, the brightest transition is S 0 → S 16 . As described in the Models subsection, the optimized structure adopts a herringbone arrangement (see Fig. ). The corresponding FTDM matrices are shown in Fig. . While both excitons are delocalized over the octamers, the delocalization pattern is different, with dominant contributions from molecules 1 and 8 in the case of the inital structure, and from molecules 3 and 7 in the case of the optimized structure. It is also seen that the CT elements are reduced for the optimized structure.