The (1, 2, 0) and (2, 2, 0) clusters differ by one ammonia, with the addition of ammonia resulting in deprotonation of one iodic acid. We see that difference reflected in the halogen bond interaction distance: in the (1, 2, 0) cluster, the hydrogen bond formed between the OH of one iodic acid and an oxygen of the other results in a more strained geometry that leads to a weaker halogen bond than in the (2, 2, 0) cluster. The addition of the second ammonium allows for a more optimal halogen bonding orientation to form, leading to a shorter halogen bond interaction and therefore a stronger halogen bond, as can be referenced in Table S1 (bond 2 versus bond 6). As discussed above, halogen bonding appears to be an important stabilizing interaction in these clusters. The experimentally determined structures of both the (1, 2, 0) and (2, 2, 0) clusters are capable of supporting one more hydrogen bond to an iodic acid OH group, but instead the formation of the halogen bond is favored.