[Pw_forum] About graphene nanoribbon input.

[Lorenzo Paulatto]

On Thursday, 27 April 2017 14:16:03 CEST Pablo Álvarez Zapatero wrote:
> Thank you for your reply Dr. Lorenzo.
> 
> As you say, my system is isolated at every direction,
> just a single finite-sized nanoribbon constructed along the x direction
> (with a definite y-width).
> 
> Then, should i use just a single k-point (gamma-point) for a 4x4x1
> supercell, in a band calculation?
> 
> And again, in a Thermal conductivity calculation, which requires 3 and 4
> order forces should I use this same input?
> 

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 
IdR @ IMPMC -- CNRS & Université Paris 6
phone: +33 (0)1 442 79822 / skype: paulatz
www:   http://www-int.impmc.upmc.fr/~paulatto/
mail:  23-24/423 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05