[QE-users] "a" parameter in SYSTEM card

[Giovanni Cantele]

Dear David,
the value of a is definitely in Angstrom (
https://www.quantum-espresso.org/Doc/INPUT_PW.html#idm258).
If with 12 A it works and with 6 A it does not work, this means that maybe
your unit cell with 6 A side is not able
to accommodate the atoms (this could be inferred only if you provided the
whole input).

Let me also point out that, as far as the CELL_PARAMETERS card is
concerned, you should (maybe) find in the output
(at the beginning) a message saying that it has been ignored. Indeed, if
you DO specify ibrav >0, the unit cell vectors
are chosen by the code with the parameters given in input (
https://www.quantum-espresso.org/Doc/INPUT_PW.html#idm224),
whereas the input from the CELL_PARAMETERS ard is allowed only with ibrav =
0.

Generally speaking, it is worth visualizing the input (using for example
XCrysDen able to directly open QE input files) before
attempting any calculation.

Giovanni

-- 

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it <giovanni.cantele at spin.cnr.it>
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: https://sites.google.com/view/giovanni-cantele/home


Il giorno sab 8 lug 2023 alle ore 13:28 <dlduran at uco.es> ha scritto:

> Dear all,
>
>   I'm starting to study a perovskite and I am a bit confused about the
> parameter "a" of the SYSTEM card.
>
>   According to the pw.x input page of QE this parameter must be in
> angstroms. I have a calculation in which, apparently, this parameter
> is in bohr:
>
> &SYSTEM
>   ibrav = 2
>   a = 12.1616
>
>   With these numbers, the calculation starts and finshes well.
>
>   I have done the simple check of changing "a" to 6.0808. This last
> value should be angstroms and it should work, according to the webpage:
>
> &SYSTEM
>   ibrav = 2
>   a = 6.0808
>
>   But, this new calculation does not finish and blows up just after
> starting.
>
>   For both of them, I have used:
>
> CELL_PARAMETERS angstrom
>   6.08 -4.10  0.00
>   6.08 -4.10  0.00
> -0.46  0.00 31.37
>
>   ...
>
> ATOMIC_POSITIONS crystal
> Pb 0.45 0.06 0.04
> Pb 0.06 0.45 0.54
>
>   ...
>
>
>   I don't know what is going on. The parameter "a" is in bohr or in
> angstroms?
>
>   Thanks for your help, David.
>
>
>
> --------------------------
> David López Durán
> Department of Physics
> University of Córdoba, Spain
> Phone: +34 957 21 20 32
> https://es.linkedin.com/in/davidlopezduran
>
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