# pulling of two graphene sheets.

Hello Everyone,

I previously asked a similar question but i have a little doubt i am not able to clear.

Let us suppose i am simulating a pull of graphene sheet by giving velocity to the top atoms, another sheet parallel to this one is held at 3.4A apart. The other sheet( which is not being pulled) is held rigidly by setting the force to zero. I am pulling the unheld grapene sheet using NVT ensemble. I am using rebo+LJ model.

My question is i want the force acting on the sheet being pulled by the other sheet due to LJ interaction alone. I use compute (sheet being pulled) group/group (sheet being held)
Is this correct? If i switch the groups as discussed earlier the sign will switch. Now i want to know if the command i am using will give correct sign as i need to know if the force
is assistive or resistive? I thinked a lot and i decided that the command i am using is right, i just want to be sure as this will totally give me opposite results if i am wrong.

If anybody can help, i would be glad.

Thanks,
Freddie.

I don't really understand your model. If you are using REBO+LJ
(does that simply mean AIREBO?), you can get the force
between the 2 sheets easily with all the forces. Not sure
why you would want just the LJ portion.

To get the total force, just use compute reduce on the forces for a group
of atoms in one sheet. Even if there are forces between the atoms
in the sheet, they should sum to 0, so the remaining force
would be due to inter-sheet forces.

Steve

Freddie,

Regarding your model, I think AIREBO is a better idea. REBO + LJ is
essentially a 3-body (so-called many-body) REBO potential plus an LJ
portion that is always on. With AIREBO, you get dihedrals (4-body) on top
of the many-body and an LJ that automatically switches on and off based on
several criteria. AIREBO reproduces the graphite-to-diamond transition
pressure, and I think it would be more appropriate for your graphene
system.

Ray

My question is i want the force acting on the sheet being pulled by the other sheet due to LJ interaction alone.

Does this mean you want the reaction force that the stationary sheet applies to the one being pulled to be only of an LJ type?

I thought by Newton’s 3rd Law the sheet held in place would try to attract the other sheet that’s tugging on it as it’s being pulled away.
However, after soon realizing that it was an LJ force and not just a simple Hooke’s law attraction your question about the nature of the reaction force
being restorative or not also got to me.

very respectfully,

Just interested in this model.

Thanks Steve and Ray.The reason I am using REBO+LJ is because I want
to extract the LJ force between two parallel graphene sheets.I cannot
do this with AIREBO.
I am pulling one graphene sheet parallel to the other to examine how
shear force due to LJ along the graphene sheet varies with respect to
pull (displacement).I am using finite length of graphene sheet under
non periodic box.
I am using compute space group/group command to extract these LJ
forces, I want to know the force due to fixed graphene sheet on the
graphene sheet being pulled and I am unsure how the direction of force
works here, as I need to know if the LJ forces are assisting or
resisting the pull.

Thanks
Freddie

The direction of the force depends on your reference (i.e. where you
stand). Assume fixed sheet is lying below the moving sheet, and LJ force
is measured from the moving sheet, then positive force normal to the
moving direction indicates a repulsive force from the fixed sheet acting
on the moving sheet. Does that sound right?

Ray

Hey Daniel,

Yes that is exactly what i intend to understand.

Freddie.

Thanks Ray, I am now starting to understand the action-reaction pair affect
and the direction of the force.

Freddie.