Comparison of a combined quantum mechanics/interatomic potential function approach with its periodic quantum-mechanical limit: Proton siting and ammonia adsorption in zeolite chabazite

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...