reported that B3LYP might yield erroneous results if there are two electronic states with essentially the same energy (Ricca et al. [@ricca]). In such cases it is known that HF calculations might favor an asymmetric wavefunction, which can lead to the consequence that hybrid functionals are more likely to have problems than generalized gradient approximation (GGA) functionals due to the inclusion of HF exchange (Sherrill [@sherrill]). For this reason we repeated our harmonic frequency calculations with the GGA functional of Becke and Perdew, i.e. BP86 (Becke [@becke88]; Perdew [@perdew]) using a single scale factor of 0.986. However, when B3LYP encounters no problems, it is usually more accurate than GGA functionals. Hence, we report results using the BP86 functional only in those cases when the B3LYP results deviate strongly in terms of intensities. Otherwise, the B3LYP data are taken.

All spectra are IR absorption spectra. Actually, the observations of the PAHs in space deal with the emission lines of the molecules. In order to be able to compare our theoretical spectra with the observed spectra, different line widths are taken for the different bands, in a similar manner as in Ricca et al. [@ricca]. The synthetic IR spectra are generated via 