Comparison is made between a combined quantum mechanics/interatomic potential function approach (QM-Pot) and its fully quantum-mechanical limit, ab initio calculation applying periodic boundary conditions. The Hartree-Fock (HF) method is combined with ab initio-parametrized ion pair shell model potential functions. The CRYSTAL code is employed for the periodic Hartree-Fock calculations. The same double-/valence triple-zeta polarization basis sets are used in both the approaches. The proton siting and ammonia adsorption in a high-silica acidic zeolite catalyst, H-chabazite (Si/Al=11, space group P1, unit cell H-AlO2[SiO2](11)) are examined. The combined QM-Pot relative stabilities and reaction energies deviate from the periodic full QM results by 4-9 kJ/mol only, which demonstrates the power of our combined approach. This conclusion is also supported by comparison of the electrostatic potential inside the zeolite pore, calculated from the periodic wave function and by the QM-Pot approach. Framework oxygen O1 is found to be the preferred proton site and on interaction with NH3 the proton is predicted to move to NH3 yielding NH4+. The NH4+ surface species is coordinated to two framework oxygen atoms, It is by 30-35 kJ/mol more stable than the neutral adsorption complex of NH3. Evidence is produced that the failure of previous periodic HF calculations to predict a stable NH4+ ion is due to the limitations of the minimum basis set used.