Table lists the computed piezoelectric coefficients (of sizeable magnitude), relative permittivity (on the diagonal elements), computed and experimental spontaneous polarization and lattice parameters for the quinuclidinium-compounds. Full set of parameters are provided in the Supplementary material (SM). The computed polarization of [F -Q]ReO 4 is almost twice that of the polarization of the two other quinuclidiniumcompounds. The fact that polarization increases with a H/F substitution is in agreement with results for other ferroelectric molecular crystals . For this system, the substitution also inverts the polarization direction. This inversion occurs because the H-F substitution is on the opposite side of the nitrogen atom involved in setting up the direction of polarization in the HQ compound.