Exact Dynamical Basis for a Fluctuating Cell Model

Cell-like models for many-body thermodynamic properties can be derived by considering the motion of a single very light particle in a classical system. Because the configuration probabilities are mass independent, the pressure and the energy calculated for such a light particle are identical with thermodynamic values. In the special case of hard spheres it is shown that the pressure from collisions is proportional to the surface-to-volume ratio of the hard sphere free volume.