Adjusting system desnity

Dear lammps users

I am trying to simulate polyethylene using airebo potential. Initially I run the system with langevin thermostat 500K. Then I try to equilibrate it at NPT. But in the initial stage of NPT itself, I get an error non-numeric pressure value. I even tried with nve ensemble but here I get atom lost error even if I use periodic boundary conditions. How is it with pbc conditions?

Now I am thinking to start the simulation with very low density so that during equilibration I can adjust system density. So, I want to know is there any command in lammps so that I can bring the system to desired density value (experimental)? Please, also suggest any software to model the polymer.

Thank you.

Rajesh