[QE-users] QE-input file clarification

[Giovanni Cantele]

Hi,
I already gave a few hints in the previous message. Generally speaking,
what always works (and is quite important especially for students)
is the "try by yourself" recipe. When help is given, then the answer to the
raised question is important to eventually go further.

1) As said in the previous message, did you check that you are displaying
the right energy window and not bands much below or much above the Fermi
energy?

2) As said in the previous message, did you check your input structure? The
point here is that if you do it by yourself, you can use all the patience
it takes to
find out possible mistakes. If others do it for you, they for sure will do
it very quickly! I did it for you very quickly, provided I made no mistakes
(which is up to you to check!), it seems to me that
if you are comparing with MgC2 structure reported here:
https://pubs.rsc.org/en/content/articlepdf/2025/cp/d5cp00253b there are
anything but negligible differences, which might
cause in turn differences in the band structure: i) bond length therein is
2.14 A, Mg-C bond I measure here is 2.823 A, is it correct? If so a 0.7 A
difference would correspond to
> 30% strain, which can turn your structure from metallic to
semiconducting, from semiconducting to metallic or even destroy it!
Moreover, the "height" therein is 2.46 A, here it seems 2.14 A (calculated
as z_Mg - z_C). Is this correct?

3) this has minor (or no) impact on your calculation, but if you expect an
hexagonal lattice, a1 and a2 should have exactly the same length (and I
would use the specific ibrav, not ibrav=1).
Also, for a perfect hexagonal lattice the x and y coordinates of K point
you chose (if correct!) must be exactly the same.

No other hint can be given, before you are pretty sure that, apart from DFT
code and basis set, you are investigating exactly the same structure.

Just as a side (and useless) notice, the band structure reported in Fig. 1
of that paper has been drawn with very few points, that would be acceptable
only if the calculation would have been
very expensive (which is not the case of a monolayer). Should I had been a
reviewer, I would have discouraged publication of such a rough plot!
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 12:23 Jodan Ebaya <jpebaya at carsu.edu.ph>
ha scritto:

> Good day everyone!
>
> Thank you Dr. Giovanni Cantele and Dr. Nicola Marzari.
>
> And regarding to the trials i've done, i use cutoff ( 60, 80, 90 and 100)
> From relax  to plotting the band structure but still the i get zero band
> gap, Below is the latest nscf(bands) input file i have used.
>
> &control
>   calculation = 'nscf',
>   prefix = 'MgC2',
>   outdir = './tmp/',
>   pseudo_dir = './pseudo/',
> /
> &system
>   ibrav = 0,
>   nat = 3,
>   ntyp = 2,
>   nbnd = 20,
>   ecutwfc = 120,
>   ecutrho = 1200,
>   input_dft = 'PBE',
>   occupations = 'fixed',
> /
> &electrons
>   conv_thr = 1.0d-8,
> /
> ATOMIC_SPECIES
> Mg  24.30500  Mg.pbe-spnl-kjpaw_psl.0.3.0.UPF
> C   12.01100  C.pbe-n-kjpaw_psl.1.0.0.UPF
>
> ATOMIC_POSITIONS {crystal}
> Mg   0.000000000   0.000000000   0.440789640
> C    0.332488607   0.664978213   0.547855180
> C    0.667511393   0.335021787   0.547855180
>
> CELL_PARAMETERS {angstrom}
>   3.181000   0.000000   0.000000
>  -1.590500   2.758000   0.000000
>   0.000000   0.000000  20.000000
>
> K_POINTS crystal_b
>   4
>   0.00000 0.00000 0.00000  40   ! Gamma
>   0.50000 0.00000 0.00000  40   ! M
>   0.33372 0.33314 0.00000  40   ! K
>   0.00000 0.00000 0.00000  0    ! Gamma
>
> Any correction and suggestions are greatly appreciated. Thank you and
> Godbless everyone.
>
> Sincerely,
>
> Jordan Lee P. Ebaya
> MS-Physics Student
> Mindanao State University
> Iligan Institute of Technology
> Iligan City, Philippines
>
>
>
>
>
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