A comprehensive search of topologically distinct local minimum structures of protonated water octamer and the classification of OH topological types

The rooted digraph is used to topologically distinguish the isomers of protonated water (PW) cluster. We generated many PW octamer geometries and obtained 134 topologically distinct geometries of the PW octamers at the theoretical level of MP2/aug-cc-pVDZ. The temperature-dependent population ratios of those isomers were calculated. Dominant structures of PW octamers vary according to the temperature. The OH bonds of PW cluster were classified into 10 topological types according to the local hydrogen-bonding network. The vibrational frequency of the same topological type of the OH bond, which is transferable in different isomers, can be used as a vibrational spectral signature.