Overall, the harmonic vibrational frequencies of the pyridine molecule are shifted by up to 20 cm -1 from the gas phase to the complexes. Within each of the three complexes, however, the frequencies of the adsorbed pyridine molecule have marginal shifts towards higher frequency (smaller than 7 cm -1 ) upon the binding to Au clusters. This is not surprising because the heavy Au atoms vibrate very slowly and have a negligible effect on the nearby pyridine molecule. For a representative structure, Py-Au 18 -V (3 Å), the numerical values are provided in Table of the SI, and information for several key vibrational modes are collected in Table .