How to record and use the old coordinates of each atoms in LAMMPS

Again, please post to the list as well.

The addforce is calculated based on the old coords for all the previous
energy minimization calculations.

Fix set/force would have to be extended to keep multiple old coords.
You should realize that if you want to keep 1000 old snapshots around,
that means each atom has to store 1000 old coords and carry it
around with it when it migrates to a new processor. Fixes can
do that, but you'd have to write a fix that explicitly does that. And
you'd need to write a fix (could be the same one) that uses those
1000 old coords to compute a force.

Steve