  We study the origin of the cooperative nature of spin crossover (SC) between
low spin (LS) and high spin (HS) states from the view point 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 Nos\'e-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.
