  In this work, we firstly investigate how to reproduce and how well one can
reproduce the Woods-Saxon density distribution of initial nuclei in the
framework of the improved quantum molecular dynamics model. Then, we propose a
new treatment for the initialization of nuclei which is correlated with the
nucleonic mean-field potential by using the same potential energy density
functional. In the mean field potential, the three-body force term is
accurately calculated. Based on the new version of the model, the influences of
precise calculations of the three-body force term, the slope of symmetry
energy, the neutron-proton effective mass splitting, and the width of the wave
packet on heavy ion collision observables, such as the neutron to proton yield
ratios for emitted free nucleons [$R(n/p)$] and for coalescence invariant
nucleons [$R_{ci}(n/p)$] for $^{124}$Sn+$^{112}$Sn at the beam energy of 200
MeV per nucleon, are discussed. Our calculations show that the spectra of
neutron to proton yield ratios [$R(n/p)$] can be used to probe the slope of
symmetry energy ($L$) and the neutron-proton effective mass splitting. In
detail, the $R(n/p)$ in the low kinetic energy region can be used to probe the
slope of symmetry energy ($L$). With a given $L$, the inclination of $R(n/p)$
to kinetic energy ($E_k$) can be used to probe the effective mass splitting. In
the case where the neutron-proton effective mass splitting is fixed, $R(n/p)$
at high kinetic energy can also be used to learn the symmetry energy at
suprasaturation density.
