For the first time, the physical nature of intermolecular interactions between 10 DNA stacked bases has been examined by an interaction energy decomposition method based on the variation-perturbation theory. The dominant correlation term including dispersion interactions is found to be cancelled to a considerable extent by exchange and delocalization contributions leaving the relatively smaller long range electrostatic contribution as the most important factor determining relative stacking interaction energies. The electrostatic multipole component closely mimics the trend of total interaction energy in all considered systems. (C) 2003 Elsevier B.V. All rights reserved.