In molecular dynamics (MD) simulations, force is the quantity needed for the simulation to progress. The force expressions can be easily obtained from the Ewald formulation, equations (3) and (4), by direct differentiation in each coordinate p=x,y,z. The resulting equations are:
The force exerted on particle i is the sum of the direct-space force, 
,
and the reciprocal-space force, 
, for all components p=x,y,z.
Note that the self-term 
in Eq.(5), is constant and hence does not contribute
to the forces in a simulation, however it adds a significant contribution to the potential.
