Molecular dynamics study of thermal expansion and compression in spin-crossover solids using a microscopic model of elastic interactions

We study characteristic features of the temperature and pressure dependences of the volume and high-spin fraction of spin-crossover solids. We develop a model and method of molecular dynamics (MD) in the isobaric-isothermal ensemble to treat the motion of the intramolecular totally symmetric mode, taking into account the entropy effect from other modes. We find several interesting observations, e.g., a nonmonotonic temperature dependence of the volume. Sigmoidal relaxations from the excited high-spin state (photoinduced state), which were often observed in experiments, are automatically reproduced in the present deterministic dynamics of MD, although it has previously been studied by assuming Arrhenius transition probabilities in stochastic dynamics.