[Pw_forum] About graphene nanoribbon input.

[Pablo Álvarez Zapatero]

Hello,
if your system is isolated, i.e. it is a finite piece of graphene, and it is
small enough to simulate it entirely in DFT, than you have to use a single k-
point, and you do not need to build any supercell. I.e. you only need a cell
bug enough to keep the ribbon in it, and ensure that there is a bit of vacuum
on every side.

Than for both phonopy and phono3py calculation, you specify 1x1x1 as a
supercell size. This will still require a huge amount of DFT calculations, as
your cell includes many atoms.

Finally, I don't know how much sense it makes to compute thermal transport for
a nanometer sized finite system. Heat clearly transfer ballistically inside the
ribbon, as it is much smaller than the mean-free-path of any phonon. More in
general: heat is a macroscopic quantity, and what does it mean to have a
temperature gradient in a single molecule of carbon, formed by just 84 atoms?

--
Dr. Lorenzo Paulatto


Dear Lorenzo,

As you say in the first paragraph, that is what i in first place tried. I calculated
a 1x1x1 supercell dimension, with a single gamma point (k_points {gamma}), and the band results
were absurd (it did not look at all alike the reference i am using). I deem it could be related to the fact
that phonon bands are supposed to be calculated in an infinite system, so with a 1x1x1 supercell that is not posible...

The idea i have is firstly perform a band calculation which results i know in advanced,
so i can try after a thermal calculation of a bigger isolated nanorribon, which is my real task. The point is that
the finite size of the nanoribbon, will play a role in thermal contuctivity as you suggest.

So, since is not a problem of k points, since i am using a single gamma point, what could be?
And, if i try to solve the phonon band problem with a 4x4x1 supercell, which k_point mesh would be convenient?

Once again, thank you for your time.

Pablo.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20170427/b82401bf/attachment.html>