We propose a new definition of complexity. The definition shows that when a system evolves to a final state via a transient state, its complexity depends on the abundance of both the final state and transient state. The abundance of the transient state may be described by the diversity of the response to disturbance. We hope that this definition can describe a clear boundary between simple systems and complex systems by showing that all the simple systems have zero complexity, and all the complex systems have positive complexity. Some examples of the complexity calculations are presented, which supports our hope.
The effect of momentum-dependent interaction on the kinetic energy spectrum of the neutron-proton ratio （ （n/p）gas）b（ Ek ） for 64Zn ＋64Zn is studied. It is found that （ （n/p）gas）b（ Ek ） sensitively depends on the momentumdependent interaction and weakly on the in-medium nucleon-nucleon cross section and symmetry potential. Therefore （ （n/p）gas）b（ Ek ） is a possible probe for extracting information on the momentum-dependent interaction in heavy ion collisions.
