Dear lammps users,

I am simulating a linear melt polyethylene structure . The structure contains 23 chains of 180 carbon atoms each.

I have modeled the system with the TraPPE-UA potential for united atoms and equilibrated it using the method of “phantom chains simulation” at 300 K and 1 atm.
After the equilibration i want to calculate the strain of my system under specific uniaxial stress so i use the command: fix 1 all npt temp 300 300 1 x -145.2 -145.2 1000 y 1.0 1.0 1000 z 1.0 1.0 1000 drag 1.
My problem is the following: The averaged pressure over time does not converge to the values i have set with the npt command.
These are the results of the pressure after averaging every 100000 steps (step=1femtosecond) for 700000 steps using the command fix 4 all ave/time 1 30000 100000 :

pxx pyy pzz (atm)

-45.9968 -48.1742 -49.0495
-49.4132 -47.4046 -47.7341
-48.6964 -47.5332 -45.7315
-47.2617 -49.2964 -46.8012
-47.4547 -47.5195 -47.2357
-48.0259 -49.923 -45.793
-48.3982 -46.9983 -47.4239

  • I am using periodic boundary conditions, units are real…

Can you use npt iso or npt anise with a single pressure
target and get the system to equilibrate properly
to that pressure? If so, there is no guarantee especially
for soft materials that you can impose radically different
pressures in x vs y,z and induce the system to sustain
that pressure difference.

Also are you using real units so that your damping
constant of 1000 = 1000 fs? I would also try this
without a drag setting first.