Simple two-dimensional model for the elastic origin of cooperativity among spin states of spin-crossover complexes.

We study the origin of the cooperative nature of spin crossover (SC) between low-spin and high-spin (HS) states from the viewpoint of elastic interactions among molecules. As the size of each molecule changes depending on its spin state, the elastic interaction among the lattice distortions provides the cooperative interaction of the spin states. We develop a simple model of SC with intra and intermolecular potentials which accounts for the elastic interaction including the effect of the inhomogeneity of the spin states and apply constant temperature molecular dynamics based on the Nosé-Hoover formalism. We demonstrate that, with increase of the strength of the intermolecular interactions, the temperature dependence of the HS component changes from a gradual crossover to a first-order transition.