[lammps-users] Carbon nanotube question

Dear Friends

1-Here, there is a problem to model my CNT while using commands like
"fix deforme" and "displace box".
I have not defined the geometry of my model (lattice, region,...)in
the input file. I have entered
the coordination of CNT as a read file and then I do not have any
control over the box and also the
box itself does not have a proportional relation to the CNT. Thus,
when I used "fix deforme" with erate=0.1,
I am not sure in which rate CNT itself expands and therefore I cannot
use "variable ex equal lx/lx0-1.0"
to find the strain because the I do not have current lenght of the CNT.

2-As I tried to apply boundary conditions to both ends of my CNT for a
uniaxial tension test, I faced
with an error saying that I cannot use this command twice. Now, I want
to ask you how I can apply the
displacement to both ends of CNT, only using "fix deforme" once?

3-Considering that one end is in the direction of x and the other one
is in the direction of -x,
unfortunately I could not find how to use "fix deforme" command to
apply displacement in -x direction.

Again, I want to really thank you for your time.


Fix deform does something simple. It deforms the simulation
box. If you use the option remap x (on by default), then it also
deforms the atoms, proportionally to the box deformation. Both
of these happen in a time dependent manner as a simulation


Hi Steve

Thanks for your answer. I want to put tension on both ends of CNT, but
there are two problems. First, when I use two "fix deform" commands, I
face with an error saying that it is not possible to use two "fix
deform" commands".Now I want to know how I can use this command to
solve my prolem. Second, How I can define tension in two opposite
directions(for example x and -x)?


It makes no sense to use 2 fix deform commands. You only
deform the simlulation box once. If you expand the box in
x and the CNT is aligned in x, then the CNT will stretch in
x as well, from both ends.