obvious from the structure of the molecule, not resulting in a Stone-Wales defect (see Fig. \[Entstehung.fig\]). The minimum activation barriers for transforming pyrene and coronene into their (5,7)-ring defected forms are found to be 8.21 and 8.41 eV, respectively. The corresponding reverse barriers are 5.92 and 6.10 eV, respectively, and are thus close to the 6.47 and 6.41 eV found for the neutral and cationic $C_{48}H_{18}$ PAH model studied in an earlier investigation (Yu & Nyman [@yu]). Hence, the energy required to build (5,7)-ring defects seems to be insensitive to the actual size of the PAH molecule which is in line with the explanation given for the high barriers in the previous study of Yu & Nyman ([@yu]). Therein, it was shown that for the transformation two $\pi$ C-C bonds have to be broken which requires a large amount of energy. The latter should obviously be quite insensitive to the size of the molecule.

![Relative energies (in eV) for the structures along the minimum energy pathway connecting defect-free and defected (a) pyrene and (b) coronene. Note that in case of pyrene two transition structures are involved (TS1 and TS2) separated by a shallow local minimum (LM), see also Fig. \[Entstehung.fig\]. 