  Within two different frameworks of isospin-dependent transport model, i.e.,
Boltzmann-Uehling-Uhlenbeck (IBUU04) and Ultrarelativistic Quantum Molecular
Dynamics (UrQMD) transport models, sensitive probes of nuclear symmetry energy
are simulated and compared. It is shown that neutron to proton ratio of free
nucleons, pi-/pi+ ratio as well as isospin-sensitive transverse and elliptic
flows given by the two transport models with their "best settings", all have
obvious differences. Discrepancy of numerical value of isospin-sensitive n/p
ratio of free nucleon from the two models mainly originates from different
symmetry potentials used and discrepancies of numerical value of charged
pi-/pi+ ratio and isospin-sensitive flows mainly originate from different
isospin-dependent nucleon-nucleon cross sections. These demonstrations call for
more detailed studies on the model inputs (i.e., the density- and
momentum-dependent symmetry potential and the isospin-dependent nucleon-nucleon
cross section in medium) of isospin-dependent transport model used. The studies
of model dependence of isospin sensitive observables can help nuclear
physicists to pin down the density dependence of nuclear symmetry energy
through comparison between experiments and theoretical simulations
scientifically.
