In order to understand the unilateral deviation of the 𝜈̅ values modelled with eq. 1a compared to the DFT frequencies in H-bonding conditions, we hypothesized that ∆𝜈̅ 𝐻𝐵 is a HB-geometry-dependent value, i.e. it depends on the HB-heavy atom distance d(C≡N---Owater/MeOH) and the HBheavy atom angle θ(C≡N---Owater/MeOH) (d and θ in Fig. ). Note that we chose heavy atom-based distances and angles instead of the C≡N---Hwater/MeOH geometry used in other work due to inaccuracies in hydrogen atom positions in MD simulations introduced by frequently used constraint algorithms; furthermore, a calibration with heavy-atoms enables comparisons with protein crystal structures, where protons are very rarely resolved. Extracting the ∆𝜈̅ 𝐻𝐵 values from Fig. and Fig. and the corresponding d(C≡N---Owater/MeOH) and θ(C≡N---Owater/MeOH) from the DFT-optimized geometries, we can visualize the ∆𝜈̅ 𝐻𝐵 geometry dependences for water and methanol HBs as 2D heat plots in Fig. and Fig. , respectively. In both cases, we observe two trends: in going from short (2.5 Å) to long (5.0 Å) distances, ∆𝜈̅ 𝐻𝐵 decreases steadily towards zero, with slightly negative values at intermediate distances (3.5 -4.0 Å) for side-on HBs (see transition from blue to dark blue to blue at angles of 70° -90° and distances of 3.0 -4.5 Å; Fig. ); at the same time, ∆𝜈̅ 𝐻𝐵 decreases while going from head-on (175°) to side-on HBs (70°).