For the dipeptides, the N -deuterated structures were considered in all vibrational calculations. The quantum-chemical calculations were automated using the PyADF scripting framework. First, the molecular structures were optimized using Turbomole and harmonic vibrational frequencies and normal modes were calculated using the SNF program. For the localization of the relevant normal modes (consisting of the CO stretch modes for the metal carbonyls and of the amide I vibrations for the dipeptides), we employed our LocVib Python package. This localization provides localized-mode coordinates as well as localized-mode harmonic frequencies, transition dipole moments, and harmonic coupling constants. The anharmonic potential energy surfaces in terms of localized modes were constructed using our Vibrations code for anharmonic theoretical vibrational spectroscopy in combination with PyADF and Turbomole. Here, we used two different approximations of the potential energy surface, either including both anharmonic one-mode and twomode potentials [2-mode, see Eq. ( )], or including only anharmonic one-mode potentials in combination with harmonic two-mode potentials [1-mode+2h, see Eq. ( )]. In both cases, we used grids of 16 points along each local mode coordinate as described in Ref. 75.