An embedded cluster model for studying zeolite-substrate interactions is described. To model the environment of the cluster and to correct for cluster termination effects, a set of formal point charges is derived by a least-squares procedure which correctly reproduce the electrostatic potential of the infinite 3-D lattice, calculated at the Hartree-Fock level. The model is used to calculate structural and energetic properties of the cluster and validated where possible by comparison with the corresponding periodic calculations.