Three different levels of shifting on a purely electrostatic potential U=1/r (top) and its force F=1/r2 (bottom), as they approach a cutoff of 9Å. The unmodified potential and force are shown in black, the SP in green, the SF in red and the SFG in blue. In b, the inset shows a magnified view of the cutoff region. Note that in both SF and SFG the energies go smoothly to zero, whereas the force goes smoothly to zero only in SFG.