[QE-users] QE-input file clarification

[Giovanni Cantele]

Hi,

first of all, I suggest checking your input structure. The input files you
attached are in html format, so
it is difficult for me to try to open the file using programs able to
visualize the structure. This is the first task to
be done before running any calculation.

Second, you said you did several trials, but you did not say which! So, it
is difficult to suggest what to change
if we do not know what you did.

Third, there is no output file, which also might be helpful in trying to
figure out what's happening. For example: did you plot
the bands "as are" or you shifted by the proper Fermi energy? Are you sure
that you are not seeing the wrong energy interval?

Fourth, after a quick inspection of your inputs (that in html format are a
mess!) , it seems to me that a 120 Ry ecutwfc is too high for a PAW
pseudopotential.
If you reduce it, then reduce ecutrho accordingly.

As far as the question about the basis set is concerned, it is a long
standing problem. With localized basis sets you reduce significantly the
computational cost
of the calculation, but you should be aware of numerical problems related
to basis set size, non orthogonality of orbitals, systems where you need
basis set functions
located not at atomic positions, and so on. A plane-wave basis set is
numerically more expensive, but allows you to control the convergence with
respect to the
basis set using a single parameter, the cutoff (ecutwfc in QE). Being a
basis set of functions which are delocalized, a side effect is that they
are non zero also in the vacuum regions
where you expect nearly zero charge density. However, this turns out to be
also an advantage, because it means that the basis set has no "spatial
dependence", that is, given the unit cell and
the cutoff(s), it is independent on the atomic species and/or positions you
put therein.


If you provide more details, we could try to understand whether or not your
calculation is wrong.
Giovanni
-- 

Prof. Giovanni Cantele
Dipartimento di Fisica "Ettore Pancini"
Universita' degli Studi di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at unina
Phone: +39 081 676910

Web page: https://sites.google.com/view/giovanni-cantele/home


Il giorno mar 30 set 2025 alle ore 09:23 Jodan Ebaya <jpebaya at carsu.edu.ph>
ha scritto:

> Good day!
>
> I am Jordan Lee Ebaya, a second MS-PHYSICS students of Mindanao State
> University Iligan Institute of Technology, Iligan CIty, Philippines.
> Currenlty i am working with my graduate research which Focus on DFT
> calculation using Qunatum Espresso.
>
> I just want to ask if my input files in QE is wrong since i already have
> done several trials and still i did not get the result i wanted. Base on
> the journal Iread that was publish on RSC, thier calcualtion shows that
> there is a 0.3eV bandgap of the Magnesium Dicarbide using Amsterdam Desnity
> Functional Simulation Package ADF-BANDS with Slater Type Orbital, Double
> Zeta Polarized Basis set. I also want to know, what are the cons and pros
> of using differenet basis sets?
>
> Attached documents are my relax, scf, nscf(bands) input file and the
> resulted bands strutre.
>
> Thank you and hoping for an answer for this inquery, this will be a great
> help for my thesis.
>
> Sincerely,
>
> Jordan Lee P. Ebaya
> *MS-PHYSICS Students*
>
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