A cyclic isomer with E conformers of FA and a hydronium ion in the middle is also very stable with a nBE of -19.4 kcal/mol as shown in Figure in the SI along with additional possible structures. Adding a molecule, namely, FA 4 W 2 H + , also forms a stable cyclic structure, with the most stable structure being a cyclic structure containing E conformers and one water molecule, with a hydronium ion in the middle as shown in Figure . The The next series is of magic number FA 5 W 1 H + . With the abstraction of one water molecule, namely in cluster FA 5 W 0 H + , the most stable structure is a cyclic structure containing three E conformers and one Z conformer (in addition to a protonated FA) with nBE of -18.9 kcal/mol. An additional stable structure composed of a cyclic structure of three E conformers and a protonated FA, with an additional E conformer external to the ring with nBE of -18.7 kcal/mol is shown in Figure in the SI. For the most stable structure of FA 5 W 1 H + , we see a significant increase in the BE, with nBE of -20.8 kcal/mol for a cyclic structure containing all E conformers and a hydronium ion in the middle. Additional stable structure with Z conformers is also very stable (nBE value of -20.1 kcal/mol) shown in Figure in the SI. In FA 5 W 2 H + cluster, the additional water molecule can not be located inside the cyclic structure and placed above as shown in Figure , which results in a significant decrease in nBE (-18.7). The results are in line with the experimental mass spectra where FA 5 W 1 H + has a large peak, followed by a drastic decrease.