Therefore, to be consistent with the H 5 O + 2 and H 3 O - 2 cases, we continue using the ωB97X functional for MD simulations. The simulated spectra for AHA -along with the experimental Ar predissociation IR spectrum 37 are presented in Figure . The experimental spectra is highly complex, and many of the peaks may be contributed by combination bands or overtones, which are challenging for classical MD methods. The key features of the spectra are the two proton transfer modes with high intensities, and CNEO-MD can capture them both. Note that although the match is not perfect with about 20 cm -1 underestimation for mode 1 and 90 cm -1 underestimation for mode 2, CNEO-MD still gives the best match to the experimental peaks among the methods that have been applied to this system. For conventional AIMD, although there is a mode at 678 cm -1 , which seems to match reasonably with the mode 1, it is actually a HCOOH deformation mode rather than the proton transfer modes (Supporting Information, Figure ). Therefore, the physical picture of the conventional AIMD is completely wrong. In CNEO-MD, we observe a small satellite peak that is slightly higher in frequency than the main peak of mode 2. Through normal coordinate analysis based on the MD trajectory, we find that this analysis cannot distinguish this peak from the main peak of mode 2 obtained from Hessian (Supporting Information, Figure ). This small satellite peak could possibly correspond to the experimental peak at around 1200 cm -1 . Another important feature of AHA -is the fundamentals of CO - 2 deformations with peaks showing up as a doublet (1685 and 1700 cm -1 ) separated by 15 cm -1 . These frequencies are between the frequency of the carboxylate antisymmetric stretch (1622 cm -1 ) 68 and the frequency of the carboxylic acid CO stretch (1818 cm -1 ), which, together with the small doublet peak splitting, is an experimental evidence for the proton being equally shared by the two formate anions. CNEO-DFT harmonic analysis predicts the doublet to be at 1771 and 1784 cm -1 . Although the frequencies are overestimated, the splitting of 13 cm -1 is consistent with the experimental results. In CNEO-MD spectrum, the two peaks merge into one because of the small splitting between them. Experimental IR spectrum is from Ref. .