All materials in the (R)-OH-Q, Hdabco, and C[NH 2 ] 3 compounds have negative d 33 values. Negative longitudinal piezoelectric coefficients are rather untypical and corresponds to a contraction in the direction of the applied electric field for the converse piezoelectric effect. Negative d 33 values have been reported for semicrystalline ferroelectric polymers such as two-phase P(VDF-TrFE) and ferroelectric liquid crystals BTAs (trialkylbenzene-1,3,5-tricarboxamides) . For these materials, the negative piezoelectric response have been attributed to extrinsic contributions, such as defects, and DFT computations of phase-pure materials have predicted positive piezoelectric coefficients . However, compounds with negative d 33 can be found in other first principles studies and was recently measured for inorganic HfO 2 thin films , with DFT calculations attributing it to an intrinsic effect .