[QE-users] SCF convergence issue after first ionic step

[Giovanni Cantele]

Hi, I do not have a precise explanation of that, but I can say that I found
such a behavior several times, in the years,
for completely different systems.
And this concerns not only the convergence of the scf step, but also the
relaxation
of the atomic positions.
At some iteration it seems that the algorithm falls in some "potential
well" and cannot exit it at all.

In this respect, it helps often just to restart the calculation from
scratch using one of the last coordinate sets found by
the pw.x, or in some cases restart the code introducing in the coordinates
even very small perturbations (e.g. delete the
digits from the fourth to the last). In other cases, you might try to
increase the smearing, even just for a few steps, mostly if
the problem is the convergence of the scf cycle.
This is just empirical, but it is the signature for tricky calculations
(where there is no parameter
to identify when a calculation is going to become "tricky"!), where the
presence of local minima or other numerically hard problems,
either with respect to the charge density or with respect to the atomic
positions, can make the convergence non as easy as expected.

Maybe, someone more expert might add some more specific hints.

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 mar 19 dic 2023 alle ore 08:49 Laurent Pizzagalli <
laurent.pizzagalli at univ-poitiers.fr> ha scritto:

> Dear all,
>
> I came across an issue I never encountered before. I performed a simple
> ionic relaxation of a W cluster. The initial scf convergence is
> achieved, the ionic forces are calculated (and are reasonable for a
> first ionic relaxation step) and the new atomic positions are obtained.
> The issue arises during the next scf calculation, which diverges in a
> few steps. However, if I start a new calculation with the updated ionic
> positions as input structure, the scf calculation runs smoothly and
> converges (but the issue occurred again for the next ionic relaxation
> steps).
> I obtained the same behavior with two different pw.x versions (6.7 and
> 7.1). I also tried to set 'pot_extrapolation = none', but with no
> difference...
>
> Do any of you have an idea about the origin (and solution) of this issue ?
>
> Thanks
>
> Laurent
>
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