Polymer Melt

I have a box size 80,80,80 in LJ units. In this box, I have 200 polymers with chain-length equals 80. I should use npt to relax my system to create a polymer melt system.
What pressure and pdamp I should use to have density equals 0.844 at the end of simulation?

Why not run with fix nvt and just choose the box dimensions so you get the desired density? Much easier to do.

Yes, and then run for a bit longer with just fix nvt so you get a properly equilibrated system.

Or you can just multiple shorter runs and use change_box in between.

Thank you so much.
What about the following commands? Do you think I can use them with a long run time?

fix 1 all deform 1 x final 0 26.66403344 y final 0 26.66403344 z final 0 26.66403344
fix 2 all nvt temp 1 1 0.3

You seem to be confusing this forum with talking to your adviser. We can give you advice about which commands to use, but how to use them is a matter of applying the information from the documentation and your understanding of the science. If we could predict the result of a simulation there would be no need to do it.

So you will have to sort this out on your own. You are a researcher, right? So please apply “the scientific method™”:

  • read all required information and make a hypothesis about how things should work (which you have already done)
  • set up a (small!) test that would prove or disprove that hypothesis
  • if it proves to be correct, continue with your work. If not, start over and find a new hypothesis.

I am so sorry about inconvenience.