If you have two groups of atoms, A and B, and

you sum the per-atom force on all A atoms, then

you effectively have the total force of B atoms on A atoms.

This is b/c any A-A forces will be equal and opposite

and cancel in the summation.

Steve

If you have two groups of atoms, A and B, and

you sum the per-atom force on all A atoms, then

you effectively have the total force of B atoms on A atoms.

This is b/c any A-A forces will be equal and opposite

and cancel in the summation.

Steve