  A known class of analytic, exact, accelerating solutions of prefect
relativistic hydrodynamics with longitudinal acceleration is utilized to
describe results on the pseudorapidity distributions for different collision
systems. These results include $dN/d\eta$ measured in p+p, Cu+Cu, Au+Au, and
Pb+Pb collisions at RHIC and LHC, in a broad centrality range. Going beyond the
traditional Bjorken model, from the accelerating hydrodynamic description we
determine the initial energy density and other thermodynamic quantities in
those collisions.
