We recommend to always use the explicit isotopic masses for best method comparison 100 . Some programs use masses averaged over natural isotopic composition by default. This may have relatively small consequences for the current low resolution vibrational spectra and systems (still up to 5 cm -1 for vibrations involving sulfur due to a ratio of 1.003 between the natural atomic mass and the main isotope mass), but is clearly detrimental when comparing to high resolution (rotational) spectroscopy in the future.