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.