Hybrid organic-inorganic materials are commonly based on an inorganic framework structure hosting organic dipolar cations-often in perovskite or perovskite-inspired structures-and they commonly feature several temperature induced phase transitions with increasing degrees of orientational disorder of the organic cations at increasing temperature. Elevated temperature may lead to either stabilisation of desired phases such as phases with high dielectric constants for increased screening of charge-carriers in photovoltaics, or to detrimental disordered paraelectric phases, in which the ferro-and piezoelectric performance is lost. Thus it is of great importance to understand the influence of molecular orientational disorder and other entropic contributions in hybrid organic-inorganic materials on their stability, phase transitions and physical properties. In this article, we focus on the hybrid ferroelectric material, TMCMCdCl 3 (TMCM = trimethylchloromethyl ammonium, Me 3 NCH 2 Cl + ) with piezoelectric performance comparable to BaTiO 3 in its pure form, and to PZT for its solid-solution with the fluorine analogue of the molecular cation (TMFM = trimethylfluoromethyl ammonium). TMCMCdCl 3 crystallises in the non-centrosymmetric monoclinic space group Cc at low temperature with ordered molecular cations, and transitions to a disordered paraelectric phase in the centrosymmetric hexagonal space group P6 3 /mmc above 400 K (Fig. ). Besides the interest in its phase transitions, the mechanism behind the high piezoelectric performance cannot be described within the conventional small-displacement limit, and is thus still poorly understood. Originally, it was suggested that ferroelastic switching was responsible, but it has been argued that this is not possible when the piezoelectric response is measured with the Berlincourt method. Instead, it was suggested that phase switching between the ground state and a metastable phase is involved. Through the last decade, several studies have proposed effective model Hamiltonians to study the order-disorder transitions in hybrid organic-inorganic materials, mainly those with cubic perovskite topology. These have been based on classical dipole-dipole interactions and strain models, Pottsand Ising-like models, or pair interactions. However, none of these have been developed solely from first principles and then applied to model a ferroelectric-to-paraelectric phase transition.