The Chl a spin-density populations in the monomer and in the [P A • • • P B ] +• dimer are qualitatively different (see Fig. ). The spin population of the monomer is very similar to those calculated earlier for the chlorin and truncated Chl a molecules in Ref. . These spin densities were largely dominated by the highest occupied molecular orbital (HOMO), transforming according to the irreducible representation of point group 2 A 2 in case of chlorin and of point group 2 A for the truncated Chl a molecule . The distribution in Fig. ) is, therefore, closely related to this single 2 A 2 -like MO. Contrary to that, the spin population in Fig. ) combines features of both HOMO ( 2 A 2 -like) and HOMO-1 ( 2 B 1like) MOs of chlorin and is, therefore, strongly mixed. This mixing can be explained by the fact that the molecular structure of [P A • • • P B ] +• was only partially relaxed, while a full structure optimization was carried out for the monomer. This assumption is supported by experimental results in Fig. ) as well as by FDE-diab calculations for a fully optimized inner pair in Fig. ). In both cases, the spin density is found to be similar to that for the monomer. The mixing observed for the dimer in vacuum is, therefore, artificial and is, indeed, caused by the limitations of the structure-optimization protocol applied.