Ab initio calculations were performed to investigate the structures, energetics, and vibrations of NH4+(H2O)(n) cluster ions at n = 0-5. Equilibrium geometries of NH4+ and NH4+-H2O are optimized at the MP2, MP4, CCD, QCISD, and B3LYP levels using the 6-31G*, 6-31G**, 6-31+G*, 6-31++G**, 6-311+G**, and 6-311++G** basis sets. The benchmark calculations indicate that using MP2 and B3LYP approaches with the 6-31+G* basis set is well suited for characterizing large NH4+(H2O)(n) clusters. The two approaches correspondingly find the existence of a number of structural isomers at n = 2-5, of which the isomer with a filled first solvation shell is lowest in energy at n = 4. The calculations further predict that, at n = 5, the lowest energy isomer contains a four-membered ring with the second-shell H2O acting as a double-proton acceptor (AA). The prediction is in good agreement with the observation of jet-cooled NH4+(H2O)(5), where a characteristic hydrogen-bonded-OH stretching absorption at similar to 3550 cm(-1) is identified for the AA-H2O molecule in the vibrational predissociation spectra (Wang, Y.-S.; Chang, H.-C.; Jiang, J. C.; Lin, S. H.; Lee, Y. T.; Chang, H.-C. J. Am. Chem. Sec. 1998, 102, 8777). In this study, in addition to energetics, how hydrogen-bonding nonadditivity influences the geometries and vibrations of these clusters is analyzed.