[Pw_forum] About graphene nanoribbon input.

[Lorenzo Paulatto]

On Wednesday, 26 April 2017 14:26:54 CEST Pablo Álvarez Zapatero wrote:
> So the question is, is this input fine? Do will solve the energy problem
> (DFT ) accurately?
> 

Dear Pablo,
if your system is isolated (i.e. not periodic) along a certain direction, than 
you should use only one k-point in that direction. If the results are absurd 
the problem is not the lack of k-points.

If you are building a supercell, then you can safely divide the numbr of k-
points in the unit cell by the number of unit cells in the supercell. This is 
perfectly accurate.

However, I do not understand really what you are trying to do. Phonopy 
computes 2-body force constants by finite differences, it does not "solve" 
anything. It then uses these force constants to interpolate the phonon 
dispersion. In order to have decent phonons in Graphene, you need to compute 
2-body force constants up to at the very least four unit cells, i.e. using a 
4x4x1 supercell. 

Incidentally this can be done with the ph.x code at a fraction of the 
computational cost.

cheers

-- 
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/
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