Global minima of protonated water clusters

Candidate global minima are obtained for H3O+⋯(H2O)n clusters with n⩽20 using a basin-hopping algorithm and an empirical, polarizable model potential. We have reoptimized the lowest minima for each system using a more accurate model and find extensive reordering of the potential energy surfaces, especially for larger n. For both model potentials a distorted dodecahedron surrounding an H2O molecule is the global minimum for n=20, in good agreement with experiment. The gap between the latter structure and the lowest minimum with H3O+ in the centre is about 10–20 kJ mol−1.