  The energy and centrality dependences of charged particle pseudorapidity
density in relativistic nuclear collisions were studied using a hadron and
string cascade model, JPCIAE. Both the relativistic $p+\bar p$ experimental
data and the PHOBOS and PHENIX $Au+Au$ data at RHIC energy could be fairly
reproduced within the framework of JPCIAE model and without retuning the model
parameters. The predictions for $Pb+Pb$ collisions at the LHC energy were also
given. We computed the participant nucleon distributions using different
methods. It was found that the number of participant nucleons is not a well
defined variable both experimentally and theoretically. Thus it may be
inappropriate to use the charged particle pseudorapidity density per
participant pair as a function of the number of participant nucleons for
distinguishing various theoretical models. A discussion for the effect of
different definitions in nuclear radius (diffused or sharp) was given.
