Here, we investigate the differences between the normal modes obtained from the partial QM and QM/MM Hessians. The partial QM Hessian is considered the reference for the partial QM/MM Hessian. The errors observed here are thus in addition to those found in the previous section. However, it is possible that there is a cancellation of errors. We look at the angular deviation between the normal modes, the absolute difference in pseudotranslational and pseudo-rotational contributions, the absolute difference in frequencies, and the relative and normalized absolute difference in IR and Raman intensities. The results of this investigation are shown in Figures and. The QM/MM geometries were obtained from a constrained QM/MM geometry optimization of the fully optimized structures in Figure . To check the similarity between the QM-and QM/MM-optimized structures, we calculated the root-mean-square deviations (RMSDs) of the nuclear positions. The RMSDs are 0.20 Å for pyridine in methanol, 0.31 Å for 1,3-butadiene in water, 0.02 Å for uracil in methanol, and 0.06 Å for propanamide in water. The structural differences are thus larger for pyridine in methanol and 1,3-butadiene in water than for propanamide in water and uracil in methanol. Since the solvent environments are frozen, the RMSDs are from the solute only. The change in geometry can be internally in the solute and displacements of the solute as a whole within the solvent cage. For formaldehyde in water, the RMSD is 0.05 Å. In the formaldehyde-water cluster in water, the RMSD increases to 0.21 Å, which includes the contribution from changes in the geometry of the two water molecules. When the water molecules in the formaldehyde-water cluster are not considered, the RMSD for formaldehyde alone is 0.05 Å. Additionally, the RMSD between the formaldehyde molecules in the two QM/MM systems is 0.04 Å.