The models considered in the present work are shown in Fig. . The geometry of the azobenzene monomer (Fig. ) was optimized in the electronic ground state using the B3LYP functional and the def2-TZVP basis set. Using this geometry, we constructed linear chains of face-to-face stacked azobenzenes (H-type chains, Fig. ). The number of molecules in the chain, N , was varied from 1 to 10, and the distance between neighbouring molecules, d, from 3.0 Å to 5.5 Å. Then we considered two-dimensional, SAM-like arrangements. First, a model geometry of upright standing molecules (perpendicular to an imaginary surface) was studied (the top view is shown in Fig. ). In this model, the rectangular unit cell with lattice parameters a = 3.5 Å and b = 6.0 Å contains one molecule. Octadecamer (6 molecules in a direction × 3 molecules in b direction) and dodecamer (4 molecules in a direction × 3 molecules in b direction) clusters of this type were studied. The dodecamer model was used in our earlier work on nonadiabatic dynamics in SAMs. Further, in earlier experimental work, the molecular orientation of the azobenzene units within SAMs has been probed experimentally by means of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and the corresponding tilt and twist angles describing molecular orientation with respect to a surface of a substrate have been reported. Specifically, in Ref. 9 the following angles defining the molecular orientation were obtained: α = 76 • , β = 30 • (the tilt angle), γ = 61 • (the twist angle); see Fig. for the definition of the angles. It was also reported that there are two molecules per unit cell in the investigated azobenzene-containing SAMs. Using lattice parameters of Ref. 9, a = 7.80 Å and b = 6.05 Å, we first constructed the SAM model shown in Fig. . In this model, the second molecule of the rectangular unit cell is obtained by translation of the first molecule to the middle of the unit cell, i.e. by vector 0.5a + 0.5b. The a direction, in turn, is chosen such that the projection of the normal to the molecular plane (green arrow in Fig. ) onto the surface (ab plane) builds angle of 30 • with the a direction. Overall there are 18 molecules in this model and we will refer to it as 18a. A similar model but containing five molecules has been studied computationally by Utecht et al. We note that the mutual orientation of the molecules within the unit cell is not directly extractable from the experiment. A possible herringbone arrangement of molecules in the SAMs has been proposed based on analysis of scanning tunneling microscope (STM) and atomic force microscope (AFM) images. 9,50 Therefore, we also considered a herringbone structure. As described in Ref. 9, the two angles α and β (see Fig. ) fix the orientation of the molecule with respect to the surface but the second molecule of the unit cell can still be rotated about the surface normal while preserving α, β, and γ angles. [In passing we note that the three angles are related as cos α = sin β × cos γ.] The second molecule was obtained by first rotating the first molecule by 60 • about the surface normal, and then translating it by vector 0.735a+0.735b. This vector was chosen such to avoid clashes with neighbouring molecules. The herringbone model constructed this way is shown in Fig. . It consists of 18 molecules in total and we will refer to it as 18b. Finally, we performed a partial optimization starting from an octamer structure shown in Fig. (which was cut from model 18a) and optimizing two molecules in the middle (the "second" molecules of the unit cells) while keeping the six perimeter molecules (the "first" molecules of the unit cells) frozen. The optimization was performed at the B3LYP+D3(BJ) 51 /def2-SV(P) 49 level, i.e. with a dispersion corrected DFT method. The optimized structure is shown in Fig. . It reveals a herringbone arrangement, but the optimized molecules change the orientation to the surface. Namely, α ≈ 71 Finally, we also considered a periodic chain model with d = 3.5 Å (like in Fig. , but an infinitely long chain) with a vacuum layer of 8.9 Å in a and 15.7 Å in b direction (see Fig. for details). For comparison we also calculated absorption of the "periodic" monomer, setting the intermolecular separation to 8 Å, similarly to Ref. 34.