In line with the former studies of our group concerning the ab initio (parameter-free) calculation of adsorption-related properties, a new simple and powerful method is presented and applied to the adsorption of N2, O2, CO, CO2, and C2H2 in zeolite Y. The calculation method is investigated and some refinements on the calculation strategy are introduced. The calculation method is based on the expansion of the interaction energy in terms of the multipole moments and polarizabilities. The molecular electrical properties were calculated at a high level on the B3LYP/cc-aug-pV(Q or 5)Z. A relationship between the van der Waals radius and the radius of an atom of the interacting molecule in a charge field is proposed. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 84: 58–69, 2001