Vibronic model for spin crossover complexes

We present quantum mechanic and thermodynamic properties of a vibronic model for the spin crossover transition. First, we focus on intramolecular aspects of spin crossover complexes, where the coupling between spin state and molecular geometry is described in a single-mode picture, accounting for the quantum mixing between low- and high-spin states resulting from high-order spin-orbit coupling. Exact and Born-Oppenheimer solutions of the molecular Hamiltonian are derived and compared, demonstrating the limits of the adiabatic approximation for spin crossover systems. In a second step, we consider the crystal Hamiltonian, including intermolecular interactions of electronic and elastic nature. The model is solved in the mean-field approximation, fully preserving the quantum description of molecular units. Thermodynamic properties of spin crossover solids, such as continuous and first-order temperature-induced transitions, are derived using realistic parameter values.