[QE-users] Convergence not achieved

[Giovanni Cantele]

Dear Saiyed,

when convergence issues show up there are several tricks you might
consider, one of these is certainly reducing mixing_beta,
another could be increasing degauss.

However, before trying any trick to cure a calculation particularly
difficult to converge, as a rule of thumb the first step is to
check your input file parameters including cut-off, k-point grid, degauss,
GEOMETRY, pseudopotentials, etc.

After a quick look to your input file I can notice:
- an incredibly huge value of degauss (1d-1 Ry = 0.1 Ry = 1.36 eV, this
would smear even the band gap of silicon I think)
- a mixing beta of 0.6, actually the advise of reducing it in cases of
difficult convergence is meant to go down to even 0.1 or 0.05
- but MOST IMPORTANTLY you specify ATOMIC_POSITIONS in crystal units, that
are usually expected to be numbers in the range
[-1,1] because in units of the lattice vectors there should be no reason to
specify the position of an atom in a unit cell far apart from the
origin. On the other hand, the numbers in your input file seem to indicate
that you're actually giving the positions in Angstrom, because they are
huge. If you try to visualize your input file using, for example XCrysDen,
you can easily see atoms at incredibly huge distances. The geometry
becomes much more reasonables if you replace "crystal" with angstrom. After
doing that, recheck the geometry (with XCrysDen) to understand if it is
what you expect or if there are still issues, before running 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 mer 28 feb 2024 alle ore 08:41 Saiyed Tasnim Md Fahim <
sfahim at usc.edu> ha scritto:

> Dear users,
> I am working with calcite in Quantum Espresso 7.1. I want to perform a
> vc-relax calculation for 3 layers of calcite but after 100 iterations
> convergence is not achieved. I changed the mixing beta but it did not work.
>
> I have attached the input file with this mail. Kindly let me know how I
> should proceed for both the convergence.
>
> Sincerely,
> Saiyed Tasnim Md Fahim,
> Ph.D. Student,
> Environmental Engineering,
> University of Southern California.
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