incremental loading on molecule using minimize

Dear all,

I am trying to simulate a molecule under quasistatic tensile loading (to stretch it by pulling at one end), whereby load steps are applied incrementally using minimize.
Therefore, I fix an atom at one end using setforce and apply a constant force to an atom at the other end using addforce.

My script for this is very short and attached below, in which I am only finding the configuration of minimum potential energy using minimize and then trying to apply one load step using minimize.
However, I do not see any changes in the configuration after the last minimization, e.g., the applied force does not seem to have any effect… different values for the force constant and minimization tolerances also did not change anything.

My apologies in advance if I am overlooking something very simple!
(I am using LAMMPS 32-bit 20150121)

Many thanks in advance,

From the fix addforce doc page:

The forces due to this fix are imposed during an energy minimization, invoked by the minimize command. You should not specify force components with a variable that has time-dependence for use with a minimizer, since the minimizer increments the timestep as the iteration count during the minimization.

IMPORTANT NOTE: If you want the fictitious potential energy associated with the added forces to be included in the total potential energy of the system (the quantity being minimized), you MUST enable the fix_modify energy option for this fix.