[Pw_forum] ecutwfc convergence

[Eduardo Ariel Menendez Proupin]

Dear Yusuf
The cutoff depends on the pseudopotential that you use, not just on the
atom. Different pseudopotentials for the same atoms have different cutoff.
Also, some properties requiere higher cutoffs than others. For example,
forces and relative energies need lower cutoff than total energy and
stress. Using higher values does not harm, just take longer time and more
computer resources.  Some programs include recommended cutoffs in the
pseudopotential, but the user is still responsible of testing it.

If you want a starting parameter for graphene, use
C.pz-vbc.UPF with ecutwfc = 60.0 ,
or
C.pbe-rrkjus.UPF with ecutwfc = 30.0 and ecutrho = 180.0

the above work reasonably well to relax the structures with fixed
cell, and to get the bands and DOS. I fou want to calculate elastic
properties or LDA+U or PBE+U, or calculate conductance, etc, etc,
these cutoffs may be too low. Yo need to do tests increasing the
values until you see no change in the property that you want to
calculate.

If you plan to adsorb other atoms (e.g. oxygen) on the graphene, then
determine cutoffs that are valid for the full system and do all the
calculations with the same parameters.


Dear Manasaje

The system is graphene with 32 carbon atoms. The cutoff is not
specified in the pseudo and I need it before proceeding to other SCF
and nscf calculations and believe, cutoff is a property of an atom in
question. So can I still go ahead with the atoms in the unit cell.
Thank you for your quick response
Yusuf


________________________________
 From: Eduardo Ariel Menendez Proupin <eariel99 at gmail.com
<http://www.democritos.it/mailman/listinfo/pw_forum>>
To: pw_forum at pwscf.org <http://www.democritos.it/mailman/listinfo/pw_forum>
Sent: Wednesday, September 26, 2012 7:18 PM
Subject: Re: [Pw_forum] ecutwfc convergence


Dear Yusufm

ecutwfc and ecutrho depend only on the hardest of the pseudopotentials
used for the atoms in the molecule or the crystal. If your molecule is
C32 and have no other elements, you can make tests with a C2 (which
does not exists, I guess), and the cutoffs for C32.

Anyway, if your are going to calculate something that it is not the
total energy or the DOS, let's say, the phonons, or the Raman
spectrum, it is reasonable that you test the effect of the cutoff
first on C2 and then on C32.
---------- Mensaje reenviado ----------
From: Yusuf Zuntu <yzunt at yahoo.com
<http://www.democritos.it/mailman/listinfo/pw_forum>>
To: PWSCF Forum <pw_forum at pwscf.org
<http://www.democritos.it/mailman/listinfo/pw_forum>>
Cc:
Date: Wed, 26 Sep 2012 03:15:32 -0700 (PDT)
Subject: [Pw_forum] ecutwfc convergence

Hi,
I need clarification on kinetic energy cutoff. I have a system
comprising 32 carbon atoms. Am I to consider total 32 carbon atoms in
convergence w.r.t kinetic energy cutoff scf calculation or the two
carbon atoms in the unit cell.
Thank you for anticipated reply.
Yusuf

-- 


Eduardo Menendez Proupin
Departamento de Química Fisica Aplicada
Facultad de Ciencias
Universidad Autónoma de Madrid
28049 Madrid, Spain
Phone: +34 91 497 6706

On leave from: Departamento de Fisica, Facultad de Ciencias, Universidad de
Chile URL: http://fisica.ciencias.uchile.cl/~emenendez

"*Padece, espera y trabaja para gentes que nunca conocerá y que a su vez
padecerán, esperarán y trabajarán para otros, que tampoco serán felices,
pues el hombre ansía siempre una felicidad situada más allá de la porción
que le es otorgada. Pero la grandeza del hombre está precisamente en querer
mejorar lo que es. En imponerse Tareas. En el Reino de los Cielos no hay
grandeza que conquistar, puesto que allá todo es jerarquía establecida,
incógnita despejada, existir sin término, imposibilidad de sacrificio,
reposo y deleite. Por ello, agobiado de penas y de Tareas, hermoso dentro
de su miseria, capaz de amar en medio de las plagas, el hombre puede hallar
su grandeza, su máxima medida en el Reino de este Mundo*".
Alejo Carpentier, El reino de este mundo, (1949).

"...man never knows for who he suffers and waits. He suffers and waits and
works for peoples he will never meet, and who at the same time will suffer
and wait and work for others who will also not be happy, as man always
desires a happiness that is beyond the lot that he is given. But the
greatness of man is precisely in wanting to improve what he is. In
assigning himself Tasks. In the Kingdom of Heaven there is no greatness to
conquer, for everything there is an established hierarchy, the unknown is
unobscured, existence is without end, sacrifice, rest, and delight are
impossibilities. Therefore, overwhelmed with hardships and Tasks, beautiful
within his misery, capable of loving in the middle of calamity, man can
only find his greatness, his full self-realization, in the Kingdom of this
World." - Alejo Carpentier
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20120926/ebaa437a/attachment.html>