Mo ր ller-Plesset ͑MP2͒ and Becke-3-Lee-Yang-Parr ͑B3LYP͒ calculations have been used to compare the geometrical parameters, hydrogen-bonding properties, vibrational frequencies and relative energies for several X Ϫ and X ϩ hydrogen peroxide complexes. The geometries and interaction energies were corrected for the basis set superposition error ͑BSSE͒ in all the complexes ͑1-5͒, using the full counterpoise method, yielding small BSSE values for the 6-311 ϩG(3d f ,2p) basis set used. The interaction energies calculated ranged from medium to strong hydrogen-bonding systems ͑1-3͒ and strong electrostatic interactions ͑4 and 5͒. The molecular interactions have been characterized using the atoms in molecules theory ͑AIM͒, and by the analysis of the vibrational frequencies. The minima on the BSSE-counterpoise corrected potential-energy surface ͑PES͒ have been determined as described by S. Simón, M. Duran, and J. J. Dannenberg, and the results were compared with the uncorrected PES.