We present an embedded cluster approach for modeling interactions in zeolites and an application of this model to the study of NH3 and NH4+ adsorption in chabazite. This model utilizes the SCREEP (surface charge representation of the electrostatic embedding potential) formalism to include an accurate description of the Madelung potential in quantum mechanical calculations. The model is validated by comparison with previous cluster, embedded cluster, and periodic calculations on this system. The importance of including the Madelung potential and geometry relaxation in zeolite calculations is addressed. After considering the effects of electron correlation, basis set superposition error, and the zero-point energy, the model yields a heat of adsorption of -170 kJ/mol for NH4+ in chabazite, in good agreement with experimental TPD data.