On top of the PBE0 reference, performing vibrational perturbative calculations with VPT2 makes the mode descriptions even worse, which, conversely, dramatically overestimates the vibrational frequencies(1300 cm -1 overestimation for mode 1 and 400 cm -1 overestimation for mode 2). This poor performance of VPT2 is not surprising since we have observed similar performance of it in H 5 O + 2 . In both cases, because of the high anharmonicity and high complexity of the underlying PES, local higher order derivative information is not sufficient for an accurate description of the whole PES, leading to the failure of the popular VPT2 method. In contrast, CNEO-DFT with harmonic approximations can much more accurately predict the vibrational frequencies of the two modes and give similar results across different functionals. Specifically, the errors are all within 60 cm -1 for mode 1 and within 100 cm -1 for mode 2 when compared to experimental results. In fact, as a highly efficient method, CNEO-DFT also outperforms the DVR results based on a 2D PES and has a similar or slightly better performance than DVR results based on a 4D PES when two additional modes are included in calculations, all of which demonstrate the high promise of CNEO-DFT in describing the proton transfer modes in larger and more complex systems.