  We demonstrate how an iterative method for potential inversion from
distribution functions developed for simple liquid systems can be generalized
to polymer systems. It uses the differences in the potentials of mean force
between the distribution functions generated from a guessed potential and the
true (simulated) distribution functions to improve the effective potential
successively. The optimization algorithm is very powerful: convergence is
reached for every trial function in few iterations. As an extensive test case
we coarse-grained an atomistic all-atom model of poly (isoprene) (PI) using a
13:1 reduction of the degrees of freedom. This procedure was performed for PI
solutions as well as for a PI melt. Comparisons of the obtained force fields
are drawn. They prove that it is not possible to use a single force field for
different concentration regimes.

Soft Condensed Matter