By means of quantum chemistry and the ABEEM/MM fluctuating charge force field, an accurate interaction potential function for -H₂O system was constructed, and then it was applied to study the structures and stabilities of hydrated ammonium clusters (H₂O)_n (n＝1～9). Analysis on the binding energies and charge distributions of the clusters shows that when n≤4, as the number of water molecules increases, tends to form linear hydrogen bonds as many as possible with water molecules until its first hydration shell is completely constructed. The clusters for 5～9 water molecules tend to form a ring or a cage around the central through the network of hydrogen bonds. The strength of H-bonds between and water molecules in its 1st hydration shell is stronger than that of the H-bonds between water molecules. The results of the ABEEM/MM model are well consistent with those from quantum chemistry calculations.

Key words: hydrated clusters of ammonium, quantum chemistry calculation, ABEEM/MM model, polarizable force field, hydrogen bond