The crystal structure of the zeolite mineral chabazite has been known since the 1950s, and that of calcined all-silica CHA was reported in 1998. A structure refinement of as-synthesized Si-CHA containing TMAda + cations and fluoride anions was published in 2003 by Villaescusa et al. In this structure, which has the rhombohedral 𝑅3 ̅ 𝑚 symmetry of the CHA aristotype, the fluoride anions are twelve-fold disordered in the double six-ring (d6r) cages (face symbol [4 •6 2 ]), and the TMAda + cations, which occupy the larger cha ([4 •6 2 •8 6 ]) cages, exhibit significant orientational disorder (Figure ). The first step in the preparation of the structure for the DFT calculations consisted in a removal of the OSDA disorder, and addition of hydrogen atoms, leading to a structure in space group 𝑅3. This structure contains four non-equivalent fluoride sites, each having three symmetry images per d6r cage (Figure ). A further symmetry reduction to space group 𝑃3 2 is necessary to reduce the number of fluoride anions per d6r cage to one (balancing the charge of one TMAda + cation per cha cage).