The BChl-like monomer has a macrocycle ring with a conjugated π system and thus exhibits strong electron correlation and multiconfigurational character. This can be seen from Table 2, where each configuration state function can be considered as an approximation of a resonance formula of the molecule. Mainly, the ground state wavefunction is represented by the closed-shell configuration with a weight of ≈ 63 %, while single excitation configurations contribute to at least 2 % and double excitation configurations to at least 11 %. The first excited state wavefunction is constituted by at least 63 % of single excitation configurations, For each dimer model, we evaluated the θ angle formed by the direction of the two transition dipole vectors and the line connecting the centers of mass (taken for simplicity as the Mg atom) of each monomer (dashed lines). Carbon, oxygen, nitrogen, magnesium, and hydrogen atoms are represented in black, red, blue, magenta, and white, respectively. at least 6 % of double excitation configurations, and at least 1 % of triple excitation configurations. Furthermore, the costly multiconfigurational treatment is justified by the partial orbital occupation of the active space depicted in Tables S8 -S10 in the Supporting Information. To further emphasize this point, we performed a single-point energy evaluation of the BChl-like model at the CC2 level of theory. Here the D1 diagnostic 61 gave a value of 0.09 and 0.17 for the MP2 and CC2 parts, respectively, which are higher than the threshold values of ca. 0.04 -0.05 indicating multiconfigurational character. These analyses are in line with previous studies on BChl molecules. Therefore, we conclude that it is essential to use multiconfigurational methods to properly describe the electronic structure of BChl-like molecules. ).