The structure report for ATZ had some uncertainty in the hydrogen position defining the tautomer, as the structure was determined from powder diffraction data. Our initial CSP assumed the 1H-tautomer, as shown in Figure . We ran CSP with the 2H-tautomer of ATZ (see SI for further details) to determine if the alternative tautomer would lead to a lower energy or closer geometric match to the experimental solution; the known structure forms hydrogen bonds between the N1 and N2 positions in the ring, so the 2H-tautomer could plausibly form the same crystal packing with the H atom shifted across the hydrogen bond. CSP with the 2H-tautomer does produce matches to the packing of the known crystal structure. However, after DFT re-optimization (PBE-GD3BJ), the energies of these structures were found to be more than 11.5 kJ mol -1 above the matches found with the 1H-tautomer (see Figure ). Thus, including the alternative tautomer of ATZ does not improve the CSP results for this molecule.