Several complexes of benzene with cations and hexafluorobenzene with anions have been optimized at the MP2/6-31++G**, B3LYP/6-31++G** and HF/6-31++G** levels of theory. Different aspects of the cation-pi interaction have been compared to those of anion-pi, including changes in the aromaticity of the ring upon complexation, charge-transfer effects using the Merz-Kollman charges and the contribution of dispersion energies by comparing the complexation energies computed at the B3LYP and MP2 levels of theory. (C) 2004 Elsevier B.V. All rights reserved.