I have a question about equilibrating a box of polymeric chains. I prepare a geometry for simulation with the experimental density of the polymer. So the box has enough molecules to mimic the density at room temperature and atmospheric pressure. However, when I start equilibrating after minimization, I find that the pressure values are ~10000 atmospheres when equilibrating with fix-NVE+langevin. Subsequently I apply NPT to bring the pressure to 1 atm, and after the equilibration of pressure, temperature and energy, the box seems to have slightly expanded in all the dimensions, which essentially reduces the density. Now I am unsure whether to use high values of pressure to maintain the original density or use a lower density at atmospheric pressure. Is there a different approach to obtain the correct density at equilibrium and room temp+atm conditions or does the reported value of high pressure indicate something else?