Reviewing the VPT2 calculations it is worth noting that extremely tight criteria for optimisation (and respectively for the DFT numerical grids) have to be used. Otherwise one risks variations in the relative zero-point vibrational energies of a few kJ/mol. But even with very tight thresholds VPT2 struggles to describe anharmonic O-H bonds, since it is based on a quartic force field built with local information. This is particularly serious for isomer 3, where the combined symmetric stretching mode of the two bridging protons is red shifted from its harmonic value by 733 cm 1 (2535 cm 1 vs 1802 cm 1 ) and exhibits a low overlap with the harmonic mode. This could be the reason for the discrepancy observed in the last isomer.