We have investigated the structure, interaction energy, electronic properties, and IR spectra of the ammonia-water cation (NH3H2O)(+) using density functional theory (DFT) and high-level ab initio theory. The ammonia-water cation has three minimum-energy structures of (a) H2NH+center dot center dot center dot OH2, (b) H3N+center dot center dot center dot OH2, and (c) H3NH+center dot center dot center dot OH. The lowest-energy structure is (a), followed by (c) and (b). The ammonia dimer cation has two minimum-energy structures [the lowest H3NH+center dot center dot center dot NH2 structure and the second lowest (H3N center dot center dot center dot NH3)(+) structure]. The minimum transition barrier for the interconversion between (a), (b), and (c) is similar to 6 kcal/mol. Most DFT calculations with various functionals, except a few cases, overstabilize the N center dot center dot center dot O and N center dot center dot center dot N binding, predicting different structures from Moller-Plesset second-order perturbation (MP2) theory and the most reliable complete basis set (CBS) limit of coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. Thus, the validity test of the DFT functionals for these ionized molecular systems would be of importance.