Doubt regarding analytical force calculations


I have a query regarding the force calculation that takes place in most of the pair potential classes. I was doing a simple energy calculation for a Cu lattice using LJ pair style with periodic boundary conditions, and found the analytical force and the final force values to be considerably different, and I can’t seem to understand why. More precisely, the fpair components computed in pair_lj_cut.cpp are different from the forces in the final dump file. It would be great if someone can explain this behaviour, and what changes happen to the forces after they are analytically calculated.


fpair is the pair force divided by this distance r, so that multiplying by deltax gives the x-direction force component and so on.