For the electronic structure method, although the B3LYP functional is used in this study as well as in previous works, it is in fact not the most accurate electronic functional. To assess the error introduced by the electronic structure method, we performed harmonic analysis using coupled cluster singles and doubles (CCSD), with the results shown in Table . Although harmonic analysis with CCSD does not account for NQEs, it can serve as a relatively accurate reference to assess errors in the electronic functional. Compared to the CCSD harmonic results, pure DFT-B3LYP harmonic analysis underestimates the hydrogen-bonded O-H stretch in Type IIa by 156 cm⁻¹, indicating that the B3LYP functional significantly underestimates the vibrational frequency. Therefore, although CNEO-CCSD is not yet developed, we anticipate that its predicted hydrogen bonded O-H vibration for Type IIa proline would be around 150 cm⁻¹ higher than the result predicted by CNEO-B3LYP, resulting in a separation from the C-H stretch region. In addition to B3LYP, we performed harmonic analysis and MD simulations with the PBE0 and M06-2X 92 functionals (Table , Figures ). For PBE0, the overall performance is highly similar to B3LYP, with underestimated vibrational frequencies relative to CCSD. A major difference is that CNEO-MD with PBE0 gives an even broader peak than that with B3LYP, which Given that the proline Type IIa hydrogen bonded O-H stretch has been unequivocally assigned, now the remaining question is: why is this frequency significantly redshifted to around 3000 cm -1 compared to the 3200 cm -1 results observed for other amino acids? A reasonable explanation was provided by