A model potential for proton hydrates, in which the system is treated
as a collection of rigid water monomers plus a lone proton, is used to
investigate H+ (H2O)n Clusters for n near the magic number of 21. Var
ious configurations which are local minima in the total energy are gen
erated. Both clathrate-like and amorphous structures are examined. In
all cases, the hydronium-like unit is four coordinated, donating hydro
gen bonds to three water molecules and interacting with the fourth thr
ough a favorable charge-dipole interaction. Both hydrogen bonding and
ion-dipole interactions play a role in determining the stability of a
given configuration. These features make the cluster energies sensitiv
e to the input potentials.