  The shell correction is proposed in the improved isospin dependent quantum
molecular dynamics (ImIQMD) model, which plays an important role in heavy-ion
fusion reactions near Coulomb barrier. By using the ImIQMD model, the static
and dynamical fusion barriers, dynamical barrier distribution in the fusion
reactions are analyzed systematically. The fusion and capture excitation
functions for a series of reaction systems are calculated and compared with
experimental data. It is found that the fusion cross sections for neutron-rich
systems increase obviously, and the strong shell effects of two colliding
nuclei result in a decrease of the fusion cross sections at the sub-barrier
energies. The lowering of the dynamical fusion barriers favors the enhancement
of the sub-barrier fusion cross sections, which is related to the nucleon
transfer and the neck formation in the fusion reactions.
