  The relativistic mean field approach by distinguishing \com{the slope of
symmetry energy} is implemented into the Lanzhou quantum molecular dynamics
transport model (LQMD.RMF). The collective flows in the isotopic nuclear
reactions are systematically investigated by the relativistic quantum molecular
dynamics model \com{with various slopes of symmetry energy}. The structure of
the directed and elliptic flows is consistent with the results of the
nonrelativistic transportation of nucleon system. \com{The directed flow
difference between free neutrons and protons appears in the midrapidity region.
The transverse momentum spectra of $\pi^+$ production is close to each other in
the nearly symmetric $^{108}\mathrm{Sn} + ^{112}\mathrm{Sn}$ system and the
neutron-rich $^{132}\mathrm{Sn} + ^{124}\mathrm{Sn}$ system. However, since
there are more neutron-neutron scatterings in neutron-rich system, the
transverse momentum spectra of $\pi^-$ production in the neutron-rich system
are higher than one in the nearly symmetric system. For a given reaction
system, the transverse momentum spectra of $\pi^+$ and $\pi^-$ production are
independent on the stiffness of symmetry energy. This leads to the fact that
the single ratio and the double ratio are independent on the stiffness of
symmetry energy. Moreover, the double ratio without the $\pi$-nucleon potential
decreases with increasing the transverse momentum. However, the double ratio
with the inclusion of $\pi$ potential increases with increasing the transverse
momentum.
