To investigate the errors introduced by the PHVA approximation itself, i.e., not including errors from QM/MM, we compare the full and partial QM Hessians as well as the frequencies, IR intensities, and Raman intensities derived using these Hessians. To keep the computational cost low, we used the small model systems pyridine in methanol, 1,3-butadiene in water, uracil in methanol, propanamide in water, and formaldehyde in water (see Figure ).