The model of crystals is constructed in which the effect of the extension under a heating or loading takes place. The main idea is a possibility to go
far from the equilibrum position, if the particles can not penetrate one through another. The typical example of particles interaction is the Lennard--Djons potential. The dynamic of one-dimensional, planar and spatial rod, given by infinite-dimensional hamiltonian systems is studed. Families of periodic solutions of these system having the effect of extension are constructed.
