Molecular dynamics simulations based on a self-consistent electronic model are performed to investigate the effect of ionization on the atomic motion of small van der Waals clusters. We find unimolecular dissociation (fragmentation) with time scales in the picosecond range. The dynamics during the relaxation process after ionization turns out to be extremely nonlinear, with fragmentation times which depend strongly on initial conditions. Our calculations show that the largest Liapunov exponent lambda(+) after ionization is much larger than lambda(0), the corresponding exponent before ionization. This indicates that the ionization process enhances the nonlinear character of the motion of small clusters. We also determined the distribution of fragmentation times as a function of the vibrational temperature of the clusters before ionization. Since the ionization process creates a state far away from thermodynamical equilibrium a time-dependent fragmentation probability W(t) is obtained. Furthermore, W(t) reflects the ionization induced chaotic dynamics. (C) 1997 American Institute of Physics. [S0021-9606(97)02747-5].