  Recently we have updated a multi-phase transport (AMPT) model with modern
parton distribution functions of nuclei (nPDFs). Here we study open charm
production in the updated AMPT model and compare to the experimental data from
$pp$ and $AA$ collisions over a wide range of collision energies. Besides the
update of nPDFs, we have removed the transverse momentum cutoff on initial
heavy quark productions and also included the resultant heavy flavor cross
section into the total minijet cross section in the initial condition as
described by the HIJING model. We show that the AMPT model with these updates
provides a much better description of the yields and transverse momentum
spectra of various open charm hadrons in comparison with the experimental data.
This lays the foundation for further heavy flavor studies within the transport
model approach.
