  Using a hadron and string cascade model, JPCIAE, the energy and centrality
dependences of charged particle pseudorapidity density in relativistic nuclear
collisions were studied. Within the framework of this model, both the
relativistic $p+\bar p$ experimental data and the PHOBOS and PHENIX $Au+Au$
data at $\sqrt s_{nn}$=130 GeV could be reproduced fairly well without retuning
the model parameters. The predictions for full RHIC energy $Au+Au$ collisions
and for $Pb+Pb$ collisions at the ALICE energy were given. Participant nucleon
distributions were calculated based on different methods. It was found that the
number of participant nucleons, $<N_{part}>$, is not a well defined variable
both experimentally and theoretically. Therefore, it is inappropriate to use
charged particle pseudorapidity density per participant pair as a function of
$<N_{part}>$ for distinguishing various theoretical models.
