  Coupled length and time scales determine the dynamic behavior of polymers and
underlie their unique viscoelastic properties. To resolve the long-time
dynamics it is imperative to determine which time and length scales must be
correctly modeled. Here we probe the degree of coarse graining required to
simultaneously retain significant atomistic details and access large length and
time scales. The degree of coarse graining in turn sets the minimum length
scale instrumental in defining polymer properties and dynamics. Using linear
polyethylene as a model system, we probe how coarse graining scale affects the
measured dynamics. Iterative Boltzmann inversion is used to derive
coarse-grained potentials with 2-6 methylene groups per coarse-grained bead
from a fully atomistic melt simulation. We show that atomistic detail is
critical to capturing large scale dynamics. Using these models we simulate
polyethylene melts for times over 500 {\mu}s to study the viscoelastic
properties of well-entangled polymer chains.
