I have been trying to implement force sampling methods in LAMMPS, which are described in the references below. I have been having some difficulty with reproducing similar results to the paper. I have written a fix that is implemented at the end of a Verlet run, and uses the forces that have been calculated previously to compute the density using the force sampling method. The system is an LJ fluid between two LJ 9/3 walls. Are there any forces that I could be missing with this implementation, or some subtle issue with this implementation that I may be missing?
Vuilleumier et. al. “Computation of pair distribution functions and three-dimensional densities with a reduced variance principle”
Schmidt et. al. “Better Than Counting: Density Profiles from Force Sampling”