The global minimum-energy and other significant structures of HCl(NH3)(n) (n=1-4) clusters have been identified by ab initio Monte Carlo simulated annealing. Geometries of the isomers were refined in density functional theoretical and Hartree-Fock plus second-order Moller-Plesset perturbation theoretical calculations. The energy orderings were confirmed in single-point higher-level calculations. While for HCl(NH3) only one hydrogen-bonded structure was found, for the larger clusters both ionic and molecular structures exist. Stabilization by hydrogen bonding is found to be less important in the ammonia clusters than in water clusters of similar size. (C) 2003 American Institute of Physics.