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[lammps-users] Young modulus of polyethylene
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[lammps-users] Young modulus of polyethylene

From: Γρηγορης Δεδες <degrigor@...447...>
Date: Sun, 27 May 2018 18:26:14 +0000 (UTC)

Hello  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.
My current work includes finding its young modulus. For that purpose i tried going step by step enforcing a standard deformation using specific uniaxial stress with 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. For some reason the averaged pressure over time does not converge to the specified values, so i cant calculate the correct averaged strain for the stress i have enforced..
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

Any suggestions concerning this matter are highly appreciated...

After the failure of this method i tried calculating the young modulus through a constant engineering strain...
I decoupled the direction x from the npt equations of the motion and i used the fixed deform command:

fix        1 all npt temp 300 300 1 z 1 1 1000 y 1 1 1000 drag 2

fix        2 all deform 1 x erate 0.00001 units box remap x

After visualizing the deformation, it seems to be working in the correct manner, however the output pressure seems to be fluctuating a lot (something reasonable for solids after reading many posts in the mail list) so i dont really have an accurate pressure value at each step of deformation..

I was wondering if there is a way to get an averaged value of the pressure at each strain value without using the npt command as i applied above or if there is a standard and more accurate way of calculating the young modulus of polyethylene..

* I am using periodic boundary conditions, units are real and the box is cubic and orthogonal...

Thank you very much in advance

Dedes Grigorios
Undergraduate Student
Nacional Technical University of Athens