Therefore, CNEO-MD seems to have captured some of the mode coupling features, although the quantitative correspondence is not great and it remains challenging for classical MD methods to accurately describe overtones, combination bands, and Fermi resonances. Apart from the proton transfer mode, there are two peaks around 1400 cm -1 corresponding to the perpendicular motions of the shared proton. Although the intensities of the two modes are low, CNEO-MD can capture them with vibrational frequencies in good agreement with prior MCTDH calculations. At around 1600 cm -1 and 1750 cm -1 , there are two water bending modes according to MCTDH calculations. The one at around 1600 cm -1 is a dark state while the intensity of the other mode around 1750 cm -1 could vary significantly in experiments depending on the type of noble gas tag. CNEO methods can accurately describe the frequencies and intensities of both modes. However, we also see a peak splitting for the bright higher energy mode, which is not expected. Using normal coordinate analysis based on the MD trajectory, we find that although the mode is dominated by water bending, it still has substantial contribution from the proton transfer motion, which can also be coupled with the water wagging motion (Supporting Information, Figure ). This may be the reason for the peak splitting. Because this splitting is not observed either in experimental or MCTDH spectra, it could be an artifact of our method. In the even higher frequency range (3400-4000 cm -1 ), we can see the symmetric and asymmetric free OH stretches. CNEO-MD describes these modes excellently, which is not surprising considering we have observed the great performance of CNEO-MD in gas phase water molecules. For the H 3 O - 2 system, Figure presents the IR predissociation spectra of the H 3 O - 2 • Ar complex and the simulated spectra for H 3 O - 2 using conventional AIMD and CNEO-MD. the IR spectrum of H 3 O - 2 is simple with most intensities on the proton transfer mode. Because DFT predicts an asymmetric placement of the shared proton, conventional AIMD gives a bound OH stretch slightly above 1800 cm -1 and thus completely fails to reproduce the experimental proton transfer mode at around 700 cm -1 . In contrast, CNEO-MD describes the proton transfer mode well with the peak position matches almost perfectly to the experimental frequency.