Abstract
The locations of calcium cations in Ca-chabazite have been explored using three different computational methods. High-silica chabazite was modelled as pure silica-chabazite with aluminium and calcium defects using empirical potential functions within the framework of the Mott-Littleton method. Higher aluminium chabazite (Si/Al=5) was explored using periodic models, first with empirical potential functions and then with first principles quantum mechanical calculations. Three sites are found, corresponding to the centre of the double-six ring (D6R) unit, in the cavity above the D6R unit, and in the eight-ring windows. The first two sites correspond closely to those determined from X-ray diffraction. In the periodic calculations the site energies tend to converge as the aluminium content increases especially for the quantum mechanical calculations as a consequence of the delocalisation of the aluminium charge around the framework. The stabilisation due to these effects is found to be more marked for the less stable sites.