Thanks Alpin,
indeed, the challenge in achieving convergence in iterative methods
(like those used in QE, VASP, Abinit, etc...) can sometimes be
(randomly) improved by changing some of these technical ingredients.
The problem to me is that iterative methods are very efficient, but when
they fail to converge it's very difficult to find a cure - and many
interesting materials, especially magnetic or mixed valence, are
difficult to converge (even most of the times). Then, one should go back
to variational methods, that are bound to converge. We found the
solution to this problem 30 years ago:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.79.1337
(also republished identically in
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.79.241103)
Only Castep and SIRIUS implemented this, but Claude now makes it easy to
implement it also in QE, so hopefully we'll see an end soon to this
convergency challenge.
nicola
On 06/07/2026 12:14, Alpin Novianus Tatan wrote:
Dear Prof. Marzari,
Thank you for your comments. I agree that if convergence is achieved and
the same physical approximation (e.g., PBE) is used, calculations
employing ultrasoft, PAW, or norm-conserving pseudopotentials should
yield the same physical results.
My previous comment was intended to address a different issue, namely
the practical difficulty of achieving convergence, which Ireneusz
encountered. In practice, the ease of obtaining convergence can differ
between pseudopotential types and computational implementations. For
that reason, I suggested trying different pseudopotential types rather
than restricting the calculations to PAW, since this may improve
convergence without changing the underlying physical approximation.
Best regards,
Alpin N. Tatan
Department of Physics
The University of Tokyo
On Mon, Jul 6, 2026 at 5:37 PM Nicola Marzari <[email protected]
<mailto:[email protected]>> wrote:
Dear all,
just a note on these comments below. All codes (all electron,
pseudopotential PAW, ultrasoft, normconserving) should give the same
numerical result for a given physical approximation (e.g., PBEsol).
If they do not, there is a problem - this is why verification (in
"verification and validation") is very important - making sure codes
give a numerically exact answer to a given physical theory (the theory
needs to be "validate", i.e. good enough to describe the physics at
hand
- you cannot do DFT PBEsol to study say a Hubbard sideband.
There has been a lot of progress on "verification", as you can see in
https://www.nature.com/articles/s42254-023-00655-3 <https://
www.nature.com/articles/s42254-023-00655-3> , with all-electron
and pseudopotential codes giving very close restuls.
For QE, there are curated efforts here:
https://legacy.materialscloud.org/discover/sssp/ <https://
legacy.materialscloud.org/discover/sssp/> - so that's a good
start, and you can find PBE and PBEsol pseudos tuned for efficiency
(lower cost) or accuracy (higher cost); we'll release very soon a major
update with all the data from the paper above.
nicola
> Authors of the paper used VASP but it should be possible in
Quantum
> Espresso.
>
>
> This is not always the case, do not assume this.
> Pseudopotentials are an important part of the calculation, which are
> different sets for VASP and QE.
> The calculation and the code implementation are also different.
>
--
----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
Director, National Centre for Competence in Research NCCR MARVEL, SNSF
Laboratory Head, PSI Center for Scientific Computing, Theory, and Data
Contact info and websites: https://theos-wiki.epfl.ch/en/Main/
Contact <https://theos-wiki.epfl.ch/en/Main/Contact>
--
----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
Director, National Centre for Competence in Research NCCR MARVEL, SNSF
Laboratory Head, PSI Center for Scientific Computing, Theory, and Data
Contact info and websites: https://theos-wiki.epfl.ch/en/Main/Contact
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