Young modulus of polyethylene

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1

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

2

Fix ave/time can be used to time average just about any
value LAMMPS produces. E.g. you can use it to get
an average pressure over a time window that corresponds
to one of your strain values.

Steve

3

Thank you very much Steve for your suggestions,

I think this is the best way to go.However, there is one more issue regarding the method you proposed that troubles me.
I try to use an engineering strain rate as small as possible (for example 10^-8) in order to get a good averaged value of the pressure around a specific strain value
(for example an average for 200000 steps). When i use a very small value of erate after some steps the y and z direction (which are barrostated)
seem to increase towards infinity…When i increase the strain value (to 10^-6 for example) the deformation seems to be happening in the correct manner

(the x direction which i have decoupled from barostat increases according to strain rate and the other too gradually decrease over time)…
Could you provide me an explanation regarding this matter?

Thanks again for your valuable advice

Στις 7:27 μ.μ. Τρίτη, 29 Μαΐου 2018, ο/η Steve Plimpton [email protected] έγραψε:

Fix ave/time can be used to time average just about any
value LAMMPS produces. E.g. you can use it to get
an average pressure over a time window that corresponds
to one of your strain values.

Steve

4

I don’t know. Assuming your 3 box lengths are all correct,
the pressure will simply go where it wants to go. If you
can do something reasonable at a high strain rate, it’s
unlikely there should be a problem with a lower strain rate.

Steve