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…