Prior to discussing the optimisation results, a brief description of the AST framework is warranted. In the cubic aristotype (space group 𝐹𝑚3 ̅ 𝑚), the T sites at the corners of the d4r cages, labelled T1, are equivalent. The d4r cages are connected to each other via linkages to another type of T atom, labelled T2 (Figure ). While the edges of the d4r cages are formed by T1-O-T1 linkages, T1-O-T1 and T1-O-T2 linkages together form the edges of larger ast (octaedecasil) cages (face symbol 4 •6 ). The T1-O-T2 angle in the cubic aristotype is fixed to 180° for symmetry reasons. Due to the instability of such straight linkages, a reduction in symmetry occurs in real AST-type materials: Whereas materials with T = Si and Ge possess 𝐼4/𝑚 symmetry, those with two different species at the T sites, like AlPO4-16, crystallise in space group 𝐼4 ̅ . When performing DFT calculations, a further reduction in symmetry is often necessary because the disorder of the OSDA molecules needs to be removed, as discussed above.