[gmx-users] How do forces from PME get calculated?

[Schlesier, Thomas]

Maybe a somewhat dumb question:
How do forces from PME get calculated?
In the manual (and also some textbooks) i have only found expressions for the potential for PME (or ewald-summation), but no information how the forces get calculated.
One idea i have would be
dF_i = - [ V(r_i(t)) - V(r_i(t+dt)) ] / [ r_i(t) - r_i(t+dt) ]
meaning, calculate the difference in the potential energy for particle i at two integration steps and divide this by the distance the particle move times (-1).
Would this be the right approach?
Thomas