pressure variation with and without restart

Dear lammps users,

I have a gold system with voids within it. I have equilibrated the system at 600 K and zero pressure by employing NPT (EAM potential) and I have no problem in achieving it. Further, I want to hold the system at constant stress (30MPa) in the x-direction and zero stress on the other directions. Following is the section of the script.

fix 1 all npt temp 600 600 0.1 x -300 -300 0.1 y 0 0 0.1 z 0 0 0.1
thermo_style custom step temp vol density pe ke etotal pxx pyy pzz lx ly lz
thermo 100
timestep 0.003
run 180000
unfix 1

Here are the observations/problems.

  1. There is a continuous increase in x-component of pressure rather than oscillating around -300 (though I am yet to figure out why there is an increase in pressure).

  2. I tried to perform the same simulation in two parts. That is, perform the simulation up to 90000 run (with the above script) and write the restart file. Followed by, the restart file is used as the initial file for the next 90000 run.
    Similar to the point ‘1’, the pressure increases up to 90000 run (Pxx=-351) and as I perform the second part, the pressure starts at Pxx=-351 (since it is stored in restart file). However, right after the first run, the pressure jumps to Pxx=-301.

I would like to know, why there is a sudden jump in the pressure value (Pxx=-301), rather than increasing from the initial value (Pxx=-351).

Kindly share your views. let me know if I am missing anything.

The restart file does not store the fix npt command itself.
In the 2nd run did you adjust the start/stop values
of your pressure targets so that correspond to
what they should be at the timesteps of the 2nd run?