Harmonic IR spectra of H 2 O -H + -NH 3 from DFT and CNEO-DFT with different levels of electron-proton correlations. Experimental IR spectrum is taken from Ref. 52 proton picture. Incorporating electron-proton correlations through the epc17-2 functional does not qualitatively alter the results, with the MAE slightly increased to 139 cm -1 . However, applying electron-proton correlations via the epc17-1 functional leads to a significant overestimation of vibrational frequencies, which raises the MAE to 443 cm -1 . Interestingly, the performance of CNEO-DFT with the epc17-1 functional is very similar to that of conventional DFT, despite the fact that the epc17-1 functional produces the most delocalized nuclear densities. In Fig. , we use the H 2 O -H + -NH 3 cluster to demonstrate the performance of DFT and CNEO-DFT on the IR spectrum of [A•H + •B] type clusters. The experimental IR spectrum shows a moderate-intensity PTM at 2649 cm -1 . Both CNEO-DFT without electron-proton correlations and CNEO-DFT with the epc17-2 functional accurately capture the frequency of this mode, while DFT and CNEO-DFT with the epc17-1 functional overestimate it. Regarding intensity, all DFT and CNEO-DFT methods predict the PTM to be the most intense peak, which contrasts with the moderate intensity observed in the experimental spectrum.