Piezoelectric properties can be very sensitive to lattice constants making the choice of exchange-correlation functional in DFT important. Figure shows the mean (absolute) deviations, M(A)D of the computed lattice parameters for the materials studied here for a set of different functionals. Reference structures are provided by low-temperature experimental crystal structures . The van der Waals (vdW) functionals, vdW-DFcx , vdW-DF2-B86R , and vdW-DF2, all provide accurate lattice constants, so does the Taktchenko-Scheffler method (PBE-TS) . PBE , which lacks an explicit account of van der Waals forces, overestimates lattices constants. SCAN+rVV10, on the other hand, underestimates the lattice constants. The largest deviations are found for the C[NH 2 ] 3 ClO 4 system, which is a supermolecular layered system, with intralayer hydrogen and intermolecular bonds in the layers, and weaker dispersion forces between the layers. PBE, SCAN+rVV10, and vdW-DF-cx resulted in c-axis lenghts of 9.48 Å, 8.17 Å, and 8.54 Å, compared to an experimental lattice constant is 8.83 Å. Further details on lattice constants can be found in the supplementary materials (SM). Based on this study, vdW-DF-cx was adopted for predicting material properties.