  In Molecular Dynamics (MD), the forces applied to atoms derive from
potentials which describe the energy of bonds, valence angles, torsion angles,
and Lennard-Jones interactions of which molecules are made. These de finitions
are classic; on the contrary, their implementation in a MD system which
respects local equilibrium of mechanical conditions is usually not described.
The precise derivation of the forces from the potential and the proof that
their application preserves energy is the object of this note. This work is
part of the building of a multi-scale MD system, presently under development.
