On 12/19/23 11:08, Laurent Pizzagalli wrote:
It seems to me that with 'pot_extrapolation=none' and 'wfc_extrpolation =none',
the scf calculation at the n+1 ionic relaxation step does not use information
from the electronic structure relaxed at ionic step n (i.e. it is
equivalent to running a scf calculation from scratch).
it is not exactly the same calculation, because
- if I remember correctly, the starting wavefunctions and charge
densities are anyway taken from the previous step
- thresholds on scf ans iterative diagonalization are computed differently
Hard to say why these rather minor differences lead to a different outcome.
Paolo
On 19/12/2023 10:29, Giovanni Cantele wrote:
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
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Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
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Il giorno mar 19 dic 2023 alle ore 08:49 Laurent Pizzagalli
<[email protected]> 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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------------------------------------------ Fax +33 549 49 66 92
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Departement de Physique et de Mécanique des Matériaux
CNRS UPR 3346
Université de Poitiers
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