In this class of submissions, we have partly anharmonically corrected values for the bonding OH stretch. AC1 makes use of the wB97xD/def2-TZVP level of theory (the harmonic results built the PH4 set), and extends the prediction by including anharmonic corrections to the OH stretch mode. This was performed by computing numerical differences along the normal coordinates. The large errors found in this particular set are a direct consequence of the procedure used. When adding an anharmonic correction AC2 and AC4 are among the best submissions for this challenge. In the case of AC2, internal potentials for the water molecule in each monohydrated system were built from B3LYP-D3(BJ)/augcc-pVTZ calculations (a functional which was already confirmed to give good results in the PH category). The latter PESs were then used as reference to respectively fit three Morse potentials for the internal degrees of freedom of water adding a Reproducing Kernel Hilbert Space (RKHS) correction term which accounts for 2-and 3-mode coupling. The vibrational states are then computed with the Discrete Variable Representation Method (DVR) and the DVR3D program package, using the water molecule geometry as in the hydrated complex. This approach lays the focus on the treatment of the water internal potential, neglecting any coupling with the organic molecule vibrations. In case strong resonance effects or even harmonic mixing with al-cohol groups would be present, this effect could not be captured, but this was not a point of concern for the set of molecules chosen in the challenge. It should be noted that the largest deviation in AC2 is found for the PYR system (36 cm -1 error), which is also where the largest experimental shift is observed. Close in performance, AC4 also focuses on the internal degrees of freedom in the water molecule, albeit with a different approach. The counterpoise corrected MP2/6-311++G(d,p) level of theory is used for both the optimisations and the potential energy calculations. Using the Quartic Force Field formulation of Yagi et al. , potential surfaces with up to 2-mode couplings were computed considering water displacements according to the normal modes computed for the complex (after identifying the modes of interest for the water). The structure of the pairing organic molecule was kept frozen. The final wavenumbers are computed from the correlation corrected VSCF method, which is described elsewhere. The agreement between AC2 and AC4 is striking, confirming the soundness of approximating the downshift by considering only the water internal degrees of freedom.