We report a semiempirical quantum mechanical study of adsorption or argon in the siliceous zeolite silicalite-1. Calculations were performed with the extended Huckel theory modified for solid state applications using crystal periodicity. The aim of this work was to derive the repulsive part of the argon-zeolite adsorption potential. We compare our results with the repulsive energy derived from two empirical potential functions for the adsorption of argon in zeolite : the effective Lennard-Jones form parameterized by Kiselev(41) and a more sophisticated potential function (called hereafter PN1) derived from the perturbation theory which underlies the general theory for intermolecular interactions.(11) We discuss our results in terms of the diameter of the channel aperture in the zeolite crystal and show that the value for the effective diameter of the zeolite channels is smaller than that usually cited, in agreement with the recent periodic ab initio results.