A decomposition analysis of the interaction energy of molecular complexes using both semiempirical (PM3) and ab initio methods shows major differences. Whereas electrostatic stabilization accounted for a significant part of the interaction by ab initio theory, the electrostatic energy in semiempirical theory was mainly repulsive. This difference has major implications for intuitive models of intermolecular interactions, particularly in light of recent AM I and PM3 energy decomposition calculations suggesting that charge transfer and polarization provides the binding energy of molecular clusters, including protein-solvent systems. (C) 2002 Published by Elsevier Science B.V.