AbstractAb initiocomplete optimizations at MP2/6‐31++G** level have been performed in the T‐shaped geometry of the benzene–benzene and benzene–naphthalene complexes. To check the effect of the basis set superposition error (BSSE), optimizations have been done in the BSSE corrected and BSSE uncorrected potential energy surfaces. The BSSE effect in the calculation of the Hessian has also been evaluated to check its influence in the frequency values. Quantum theory atoms in molecules (QTAIM) calculations have also been performed on both dimers. Intermolecular energies differ around a 25% when the optimization is performed with or without counterpoise corrected gradients. The influence of BSSE is also noticeable in the distances. Frequency shifts show big changes because of the BSSE. Thus, uncorrected values are up 350% larger than corrected ones. The hypotheses given in the literature to explain the origin of the blue‐shifting hydrogen bond do not seem to give a suitable explanation for all characteristics of the behavior found in the studied systems. © 2006 Wiley Periodicals, Inc. J Comput Chem 28: 540–546, 2007