The final and largest cluster to be discussed is the (2, 2, 1) cluster. Figure displays the three lowest energy calculated spectra compared to the experimental spectrum. Figure in the supplementary information contains all of the structures and calculated spectra explored at this cluster size. The experimental spectrum contains a single free OH stretch, and now only contains a single free NH stretch unlike the smaller clusters of this class. The single, somewhat broad NH stretch suggests that both ammoniums are likely triply hydrogen bonded and mostly symmetric. There is a broad tail on the lower wavenumber side of this peak, which is presumably due to tag effects, as that bump does not appear in the infrared multiphoton dissociation (IRMPD) spectrum in the SI (Figure ). There is also a very small free OH stretch peak (highlighted in red) in the experimental spectrum red-shifted from the more prominent, labeled free OH stretch that is also likely the result of tag isomers (tag on OH versus tag on NH). The IRMPD spectrum of this cluster does not contain this peak, further supporting this claim.