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[lammps-users] PRESSURE DOES NOT CONVERGE TO SET VALUE
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[lammps-users] PRESSURE DOES NOT CONVERGE TO SET VALUE


From: Γρηγορης Δεδες <degrigor@...447...>
Date: Tue, 22 May 2018 18:20:34 +0000 (UTC)

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..