  We have updated the parton and hadron cascade model PACIAE 2.0 for
relativistic nuclear collisions (cf. Comput. Phys. Comm. 183(2012)333) to the
new issue of PACIAE 2.1. The PACIAE model is based on PYTHIA. In the PYTHIA
model, once the generated particle/parton transverse momentum $p_T$ is randomly
selected, its component of $[p_x,p_y]$ was originally put on the circle with
radius $p_T$ randomly. It is now replaced by put on the circumference of
ellipse with half major and minor axes of $p_T(1+\delta_p)$ and
$p_T(1-\delta_p)$, respectively, in order to investigate better the transverse
momentum anisotropy in final hadronic state.
