  We present an extensive set of simulation results for the stress relaxation
in equilibrium and step-strained bead-spring polymer melts. The data allow us
to explore the chain dynamics and the shear relaxation modulus, $G(t)$, into
the plateau regime for chains with $Z=40$ entanglements and into the terminal
relaxation regime for $Z=10$. Using the known (Rouse) mobility of unentangled
chains and the melt entanglement length determined via the primitive path
analysis of the microscopic topological state of our systems, we have performed
parameter -free tests of several different tube models. We find excellent
agreement for the Likhtman-McLeish theory using the double reptation
approximation for constraint release, if we remove the contribution of
high-frequency modes to contour length fluctuations of the primitive chain.
