The normal modes of both uracil in methanol and propanamide in water have large frequency errors. Especially modes 1, 2, and 18 in uracil in methanol and modes 1, 2, and 21 in propanamide in water have errors close to or above 100 cm -1 . These large errors seem to be related to the fact that all of the modes include vibrations of an N-H moiety that is involved in hydrogen bonds to solvent molecules. Hydrogen bonds crossing the QM-MM boundary are only partially described within the QM/MM model. This is also reflected through the change in the N-H bond length that increases by 0.015 Å for uracil in methanol and 0.017 Å for propanamide in water when going from full QM to QM/MM, indicating too strong hydrogen bonds. Additionally, the frequency errors are likely made worse by basis-set superposition errors in the reference calculations. On the other hand, the normal modes that include C=O stretching, i.e., modes 5 and 6 in uracil in methanol and modes 8 and 9 in propanamide in water, which are also involved in hydrogen bonds, have relatively low errors.