The Al sites of interest were modeled by idealized truncated molecular species containing up to 4 monodentate (OCUBE = (Me 3 Sn) 7 Si 8 O 19 (O-)) or 2 bidentate (O 2 CUBE = (Me3Sn)6Si8O18(O-)2, bound through neighboring corners) CUBEs. The terminal -OSnMe 3 groups of the CUBEs were replaced by -OMe for computational efficiency (fewer nuclei and electrons) and the effect of the substitution was benchmarked. The molecular geometries were optimized by the PBE0 functional and the def2-TZVPP basis set (the central part of the molecule, e.g., THF-Al(OSiO 3 ) 3 , up to and including the first [SiO 4 ] tetrahedron) or def2-SVP basis set (the rest of the molecule) with the corresponding def2-ECP for Sn atoms as implemented in the Turbomole 7.5.0 program. The structures were optimized using the DFT-D3 dispersion correction with Becke-Johnson damping and the Conductor-like Screening Model (COSMO) solvent model of THF. For the calculations of chemical shielding, the central part of the molecule was taken out (e.g., THF-Al(OSiO 3 ) 3 ), terminated by hydrogen atoms (e.g., 9 H to form THF-Al(OSi(OH)3)3) and reoptimized with frozen coordinates of the central atoms. All calculations using this protocol were performed with the m5 integration grid and the following convergence criteria: 10 -6 for the density matrix change and 10 -4 Hartree Bohr - 1 for the geometry gradient.