The standard density functional approach used to obtain approximate structural and energetic predictions for the systems of interest in this work is three-body-inclusive D3-dispersioncorrected B3LYP with the def2-TZVP basis set in the harmonic approximation, in short B3LYP/TZ. The results were mainly obtained using ORCA 4.2.1 , with some interim ORCA 5.0.3 saddle point optimisation assistance (see the ESI † for small numerical differences), always after a CREST 2.10.2 structure search . The employed keywords are listed in the ESI †. For selected structures, electronic energies with the def2-QZVP basis set were obtained (B3LYP/QZ). To interpolate between the spectral features of the different species, a uniform scaling factor is derived from a number of related closed-shell species (see ESI †). For the parent hydroxylamine, the OH stretching vibration (exp./theory = 3650/3789) and the two NH stretches (3359/3506, 3294/3428) point at a slightly smaller gas phase scaling factor (0.958-0.963) than the two isolated water stretches (3756/3888, 3657/3782), which require multiplication by 0.966-0.967. The ratio for isolated TH (vide infra) is (3631/3794)=0.957. Therefore, a rounded uniform scaling factor of 0.96 is used in all comparisons between DFT prediction and experiment. The rounding accounts for ±20 cm -1 uncertainty, probably encompassing typical residual electronic structure and anharmonic errors for OH stretching fundamentals in weak to moderate strength hydrogen bonds. For the monohydrate of hydroxylamine, only matrix isolation data are available. Due to the matrix-induced shifts, they are less useful to derive a gas phase scaling factor. For the hydrogen-bonded OH stretching fundamentals, the values range between 0.961 and 0.973 for Ar and N 2 matrices. Taking into account the scaling factor for matrix-embedded hydroxylamine itself, which is 0.959 instead of 0.963 in the gas phase, 0.96-0.98 is the expected scaling factor range for the hydrogen-bonded OH groups of the monohydrate. The uncertainty of ±40 cm -1 is too large to be useful for safe gas phase assignments.