Galaxy clusters are the largest collapsed structures in the universe and their abundance is exponentially sensitive to the growth of structure. The cluster abundance probes the gravitational aspects of dark energy as well as modified gravity explanations for cosmic acceleration. Here the impact of the f(R) enhancement of gravitational forces is shown for the density field and potential. On the right the field itself is shown. The deep graviational potentials of clusters drive non-linear effects that can suppress the force deviations and make such models compatible with local tests of gravity.