  We present a computationally efficient multiscale method for preparing
equilibrated, isotropic long chain model polymer melts. As an application we
generate Kremer-Grest melts of $1000$ chains with $200$ entanglements and
$25000$-$2000$ beads per chain, which cover the experimentally relevant bending
rigidities up to and beyond the limit of the isotropic-nematic transition. In
the first step, we employ Monte Carlo simulations of a lattice model to
equilibrate the large-scale chain structure above the tube scale while ensuring
a spatially homogeneous density distribution. We then use theoretical insight
from a constrained mode tube model to introduce the bead degrees of freedom
together with random walk conformational statistics all the way down to the
Kuhn scale of the chains. This is followed by a sequence of simulations with
carefully parameterized force-capped bead-spring models, which slowly introduce
the local bead packing while reproducing the larger scale chain statistics of
the target Kremer-Grest system at all levels of force-capping. Finally we can
switch to the full Kremer-Grest model without perturbing the structure. The
resulting chain statistics is in excellent agreement with literature results on
all length scales accessible in brute-force simulations of shorter chains.
