About the density of Polyethylene film

Hello, all,
I want to build a Polyethylene film. simulated a tip scratch on the PE film. First the density of PE film reach 0.90 used the command of “fix wall/lj126” and NVT dynamics at 500 K , then cooled the PE film down to the desired temperature(100K or 250K etc). the boundary is p p s. but the density of PE film remain unchanged, in fact, the density should be increased (more than 0.90).
Meanwhile, I want used “fix npt” ,but the boundary must be period. can you give me some advice. thank you!
shengpeng zhan

Unless you adjust the size of the box

or the positions of the wall, the density cannot

change since rho = N/V and you are fixing V.

You can use NPT on periodic dimensions, just

not on non-periodic dimensions. The wall command

also has options to change the position of the wall

versus time. You should not use a wall and NPT

in the same dimension.