Comparison between harmonic (top panels) and anharmonic (bottom panels) IR spectra of coronene (left panels) and circumcoronene (right panels). The harmonic spectra are downloaded from NASA Ames PAH Database, and multiple scaling factors are multiplied to correct the anharmonic effects (Bauschlicher et al. 2018). The anharmonic spectra are calculated at 300 K with semiempirical method of PM3. The insets show the region of combination bands, where anharmonicity and resonances dominate.