  We perform molecular dynamics simulation of a small number of particles in a
box with periodic boundary conditions from a view point of chaotic dynamical
systems. There is a transition at a critical energy E_c that each particle is
confined in each unit cell for E<E_c, and the chaotic diffusion occurs for
E>E_c. We find an anomalous behavior of the jump frequency above the critical
energy in a two-particle system, which is related with the infinitely
alternating stability change of the straight motion passing through a saddle
point. We find simultaneous jump motions just above the critical energy in a
four-particle system and sixteen-particle system, which is also related with
the motion passing through the saddle point.
