smallest one. Previous work on (5,7)-member ring defects (Yu & Nyman [@yu]) has raised the suspicion that effects of such defects might be more intriguing with decreasing size of the PAHs. To study these small systems is thought to be an important step towards a substantial understanding of the role of (5,7)-ring defects for the IR spectra of PAHs. Another peculiarity of these two molecules appears by investigating the minimal energy pathway between the ground and (5,7)-ring defected states, see section 3. Doing so, it turns out that coronene proceeds via a Stone-Wales transition (Harris [@harris]) from its ground to its defected state, but shows no Stone-Wales defect in the defected form. On the other hand, pyrene shows no Stone-Wales transition, but instead the defected pyrene exhibits a Stone-Wales defect. The latter is the underlying reason for the third respect in which the two studied molecules can be regarded as limiting and prototypical. Whereas pyrene retains some of its symmetry due to the symmetry exhibited by the Stone-Wales defect itself, coronene loses much more of its symmetry (see Fig.\[molecules.fig\]).

This work is structured as follows. In section 2, we give a short description of the methodology and the computational methods 