  Pseudorapidity distributions of charged particles emitted in $Au+Au$,
$Cu+Cu$, $d+Au$, and $p+p$ collisions over a wide energy range have been
measured using the PHOBOS detector at RHIC. The centrality dependence of both
the charged particle distributions and the multiplicity at midrapidity were
measured. Pseudorapidity distributions of charged particles emitted with
$|\eta|<5.4$, which account for between 95% and 99% of the total
charged-particle emission associated with collision participants, are presented
for different collision centralities. Both the midrapidity density,
$dN_{ch}/d\eta$, and the total charged-particle multiplicity, $N_{ch}$, are
found to factorize into a product of independent functions of collision energy,
$\sqrt{s_{_{NN}}}$, and centrality given in terms of the number of nucleons
participating in the collision, $N_{part}$. The total charged particle
multiplicity, observed in these experiments and those at lower energies,
assumes a linear dependence of $(\ln s_{_{NN}})^2$ over the full range of
collision energy of $\sqrt{s_{_{NN}}}$=2.7-200 GeV.
