The first such system was pyrene (C 16 H 10 ), a four-ring polycyclic aromatic hydrocarbon, for which the experimental gas-phase IR spectra were available on the NIST Chemistry WebBook. The comparison of the harmonic spectra with fundamentals only and with overtones and combination bands is shown in Figure . We can see that the same scale factors can be used to adjust the positions of the overtones and combination bands while no anharmonic corrections are applied. The s abs demonstrates the best performance for the C-H stretching vibrations (above 2000 cm -1 ), while the s rel performs best at the fingerprint region below 2000 cm -1 , accounting for a generally better representation of the spectral features.