  The long time dynamics of polymeric materials has been extensively studied in
the past through various experimental techniques and computer simulations.
While computer simulations typically treat generic, simplified models,
experiments deal with specific chemistries. In this letter we present a
hierarchical approach that combines atomistic and coarse-grained simulations to
quantitatively study polymer dynamics. As an example we predict diffusion
coefficients of atactic polystyrene melts of molecular weights relevant to
polymer processing (up to 50kDa) without any adjustable parameter and compare
the results to experiment.
