The ground-state properties of the charge tranfer complexes formed between NH3 and the halogens F-2, ClF, and Cl-2 were investigated systematically with the aid of ab initio calculations. Extended basis sets were applied and electron correlation was included at different levels of Moller-Plesset perturbation theory and coupled cluster expansions. Not only the level of electron correlation and the basis Set chosen are important, but also the use of the counterpoise correction to the basis set superposition error directly in the course of the geometry optimization has a significant influence on the calculated equilibrium structures. The trends in other ground-state properties of these complexes, such as vibrational spectra, infrared intensities, selected harmonic force constants, dipole moments, and nuclear quadrupole coupling constants are discussed and compared to experimental data. The strength of the intermolecular interaction in this series increases in the order NH3-F-2, NH3-Cl-2, and NH3-ClF.