The physical nature of the catalytic activity of Si-->Al substitutions in ZMS-5 zeolite has been investigated for propylene protonation reaction using hybrid variation-perturbation decomposition of intermolecular interaction energy. The differential transition state stabilization energy is dominated by long range electrostatic multipole term, whereas electrostatic penetration, exchange and delocalization terms are non-negligible for intermolecular contacts shorter than 3.0 Angstrom. Observed sensitivity of activation barrier lowerings to the kind of terminating groups could be considerably reduced by constraining corresponding torsional angles. (C) 2002 Elsevier Science B.V. All rights reserved.