A perturbational approach (M-method) for the calculation of the interaction energies of N(2), O(2), Ar and CO in zeolite cavities is presented. The calculation method is compared with the 'molecule in point charge environment' calculation and a full ab initio calculation tested along an axis connecting the center of the cavity with a site II cation. The Henry constants, heats of adsorption and separation constants for N(2), O(2) and Ar in a NaY zeolite are obtained using the 'molecule in point charge environment' method and the present approximation. Different grid sizes were considered depending on the distance between adsorbing system and cation. A promising result, in particular for the separation constants. was found using the Van der Waals and ionic radii for the grid delimitation and the M-method calculated on the B3LYP/6-31G* level. CO was added as a final test system with lower symmetry. (C) 2001 Elsevier Science B.V. All rights reserved.