Starting structures of the zeolite frameworks were taken from the experimental data of all-silica or high-silica zeolites, preferentially in the calcined form (see Supporting Information, S2, Table ). Only zeolites with pores consisting of 10 membered rings of T atoms (MR) or larger were considered, since typical PPCP molecules are too large to diffuse into smaller pores. The structures were optimized using the GULP program package utilizing the SLC potentials, combining a fixed-pressure optimization with a calculation of the phonon dispersion curve. For MFI, FER, FAU, IFR, MTW, and JZT, no phonons with imaginary frequencies were observed, and the GULP-optimized structures were used in the further simulations. For the remaining 8 frameworks, imaginary frequencies were observed. For these structures, short Ab Initio MD simulations were run (DZVP-MOLOPT-SR basis set/rev-vdW-DF2 exchange-correlation functional/CP2K code) for a temperature of 298 K, using a timestep of 0.5 fs. In keeping with previous work , the average atomic coordinates were computed, using the last 10 ps of the total 12.5 ps trajectory. This structure was then subjected to a symmetry search, which typically resulted in a reduction in space group symmetry with respect to the initial (experimental) structure. The structures were then again optimized using GULP and SLC potentials. For 7 of the 8 cases, the absence of imaginary frequencies in the lower-symmetry structure could be confirmed (weakly imaginary frequencies at the Brillouin zone boundary remained for BEA, indicating that a larger cell would be necessary to remove this artefact). A comparison of the experimental and the optimized cell parameters and symmetries is provided in the SI, as well as the GULP-optimized structures employed in the further simulations. ] FD JZT ZEO-3 16MR, 3D 8.6 8.0 12,470 12.8 DON UTD-1 14MR, 1D 7.9 7.3 22,098 17.4 CFI CIT-5 14MR, 1D 6.8 6.6 14,090 18.2 FAU Dealum. Y 12MR, 3D 10.6 6.8 14,219 13.5 AFI SSZ-24 12MR, 1D 7.1 6.7 15,949 18.1 IFR ITQ-4 12MR, 1D 6.5 5.9 11,184 17.2 BEA Beta 12MR, 3D 6.1 5.5 16,297 15.7 MOR HS-MOR 12MR, 1D 5.9 5.7 10,738 17.9 MTW ZSM-12 , dealum.