How to fix the position of graphene sheet in simulation box

Dear Sir,

In my simulation box, polymer, water molecules and graphene sheet are there. During simulation running, I want to fix the movement of the graphene sheet in the simulation box.
Please suggest some ideas

This is a bad choice of words in the context of LAMMPS since it has a “fix” command. You don’t want to “fix” those atoms (you probably mean “fixate”) since that means in the context of LAMMPS “fix up” (or “modify”) but rather you want to immobilize them.

There are multiple ways to immobilize atoms. The simplest is to not include them in the group to which time integration is applied.

This topic has been discussed many times, so please search the archives for detailed discussions.

Thanks Sir

Dear sir,

I followed the same concept as discussed in previous topics .
group mobile_gr subtract all fx_gr
fix 3 mobile_gr nvt temp 300.0 300.0 100.0

When I apply this same concept in npt , I am facing one problem.
that is…
In my simulation box, polymer + graphene sheet + water molecules . During NPT run, polymer is aggregating, but water is not aggregating.
I am attaching one figure of simulation box.

This is not a LAMMPS issue but a question about how to set up and monitor a simulation, how to assess what happens, and how to adjust settings and model if needed. That is a topic for discussion with your advisor or tutor or experienced colleagues, but not for a forum about the simulation software. The software will do just what you tell it to do.