[EXTERNAL] kspace_modify slab under tinfoil boundary conditions

You can also verify LAMMPS gives the correct behavior for slab systems yourself by comparing to an analytic solution. Just place two oppositely charged particles in a slab system. They form two infinite sheets of charge due to periodic boundary conditions in the x and y directions. If the charges are far apart enough in the z-direction, the force in the z-direction converges to 2piq^2/(Lx*Ly), (see Understanding Molecular Simulation, Frenkel and Smit, p. 318), and LAMMPS reproduces this result only if you include the correction factor given in Eq. 12 of the paper.