[Pw_forum] About graphene nanoribbon input.

[Pablo Álvarez Zapatero]

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?

Thank you again for your time and patience.

Pablo.

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



________________________________
De: Pablo Álvarez Zapatero <pabloalvazapa at hotmail.com>
Enviado: miércoles, 26 de abril de 2017 14:26
Para: pw_forum at pwscf.org
Asunto: About graphene nanoribbon input.


Hi everyone,


I am trying to calculate thermal conductivity of a graphene nanoribbon throught other program called "phono3py", which uses QE to perform ab initio calculations. Then, in order to understand and validating my results, i am currently trying to calculate the phonon bands of a different sized graphene nanoribbon, using the so called phonopy.

Firstly, i succesfully solved the infinite (phonon bands) grapehene system with phonopy+QE. Now, I have looked for propper inputs in QE forums for a nanorribon, but i do not know if the input i am using is well posed. Using a single gamma k point (as i read in this forum), results are ridicolous. Now, using these k-points it lasts for 24 hours for a sigle .in calculation (there are 11 more in a band calculation).


&control
 calculation = 'scf'
 prefix='grapheneph',
 tprnfor = .true.
 tstress = .true.
 pseudo_dir = './',
 outdir='./saleph51/'
/
&system
 ibrav = 0,
 nat = 84,
 ntyp = 1,
 occupations = 'smearing'
 smearing = 'm-p'
 degauss = 0.01
 ecutwfc = 30.0,
 ecutrho = 150.0,
/
&electrons
    mixing_beta= 0.7
    conv_thr = 1.0d-8
    diagonalization = 'david'
    mixing_mode = 'plain'
/
K_POINTS automatic
12 12 1 0 0 0
!    ibrav = 0, nat = 84, ntyp = 1
CELL_PARAMETERS bohr
   29.8200000000000003    0.0000000000000000    0.0000000000000000
    0.0000000000000000   16.1490000000000009    0.0000000000000000
    0.0000000000000000    0.0000000000000000   10.0000000000000000
ATOMIC_SPECIES
  C   12.01070   C.pbe-rrkjus.UPF
ATOMIC_POSITIONS crystal

[...]



I read that is convenient to use a multiple of 3 k-point mesh to achieve better results in any direction (i read for instance 12 or even 60!). Since my system is flat, i set that k-point mesh in x-y.



So the question is, is this input fine? Do will solve the energy problem (DFT ) accurately?


Thank you for your time and patience!

Pablo.
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