[lammps-users] Discrepancies in simulating failure strain for CNT

I tried performing a failure strain simulation for a simple carbon nanotube(CNT) using AIREBO potential . I fixed one end (along the axis of the CNT) and pulled the other end with a constant velocity. I performed the simulation once at temperatures 1K and other at 300K. Irrespective of the temperatures I am getting a failure strain of around 0.36 which according according to various established experiments and research papers should be less than 0.20. Can anyone suggest where I did go wrong ?? Also while stretching the diameter of the CNT should decrease as it elongates axially but I am not getting this.

I tried performing a failure strain simulation for a simple carbon
nanotube(CNT) using AIREBO potential . I fixed one end (along the axis of
the CNT) and pulled the other end with a constant velocity. I performed the
simulation once at temperatures 1K and other at 300K. Irrespective of the
temperatures I am getting a failure strain of around 0.36 which according
according to various established experiments and research papers should be
less than 0.20. Can anyone suggest where I did go wrong ?? Also while

have you checked how much the result depends on the pulling velocity?
what kind of potential do other simulation papers use? also AIREBO?

stretching the diameter of the CNT should decrease as it elongates axially
but I am not getting this.

how large/long is your CNT?
have you checked whether the diameter changes,
if you make the CNT longer, or use a different chirality,
or a different pulling velocity?

axel

Dear Axel,
Thanks for the reply. I have checked the diameters through VMD simulation and they do not appear to decrease.
I am also mailing you my input script , please have a look at it and suggest why I am getting such high failure strains and no change in diameter.

simulate 88x5 cnt

units real
dimension 3
boundary p p p
atom_style full
newton on

processors * * 1

read_data structure_88x5.dat

group ebc id 1 2 5 6 9 10 13 14 17 18 21 22 25 26 29 30
group moving id 643 644 647 648 651 652 655 656 659 660 663 664 667 668 671 672
group mobile subtract all ebc moving
group tot union moving mobile

pair_style airebo 5.0 1 1
pair_coeff * * CH.airebo_real C

neighbor 3.0 bin
neigh_modify delay 0 every 1 check yes

timestep 1

minimize 1.0e-4 1.0e-6 100 1000
log log.cnt

lattice sc 1
velocity mobile create 1.0 46659 mom yes rot yes dist gaussian
velocity ebc set 0.0 0.0 0.0
velocity moving set 0.0 0.0 0.0005

reset_timestep 0

fix 1 tot nve
fix 2 moving setforce 0.0 0.0 0.0

thermo 1000
thermo_modify lost warn

dump 1 all custom 10 dump.lammpstrj id type x y z

run 50000

Dear Axel,
Thanks for the reply. I have checked the diameters through
VMD simulation and they do not appear to decrease.
I am also mailing you my input script , please have a look at it and suggest
why I am getting such high failure strains and no change in diameter.

i don't have the time do _your_ work.

i already gave you a number of questions,
please check them out for yourself.

this is the way how research works:
you have to first _convince_ yourself
that everything you did is correct. you
can corroborate your findings with published
results in the literature, you can do series
of test calculations on how certain input
parameters will impact the results.
this has to be done methodically, and thoroughly
and _not_ by just asking some random person
on the net.

the first thing that i would check is your pulling velocity.
also, you seem to have translated the AIREBO input to
real units, have you confirmed that this was done correct?

axel.