I am highly grateful for your so kind reply to my question.
Actually, in my simulation, I am using “fix smd cvel” to push down a rigid body which is compressing a liquid layer. And the liquid layer is subject to the constraint between the tethered body and another fixed substrate. I put the reference point of the spring always above all of these.
In the whole process, I need frequently stop and restart my simulation run to see the change of the liquid layer and do the equilibrium in between. But the restart file in LAMMPS would not store the list of fixes. So I need catch the spring force before each restart and redefine the fix smd with this force. Otherwise, the spring force would go back to zero immediately in the new run. Sometime, the spring force is positive, while sometime is negative. That’s why my question arose.
Do you have any good idea to handle this question? You kind help would be most appreciated.