[Pw_forum] convergence NOT achieved after 100 iterations --- related question

[Nicola Marzari]

Hi Hande,

first question - would mp work?

In terms of "structure" of the smearing function, gaussian is
the smoothest, then mv, then mp. So if there were an intrinsic problem
with states at the fermi energy, it should be even more apparent in
mp .

As a general comment, the role of smearing is to help improve the
accuracy of k-point sampling - and so the 0.01-.02 you are using
is exactly in the right ball park.

A side effect (a positive one) for smearing is to help convergence
in iterative algorithms (as in PWSCF) - for that side effect,
0.01-0.02 is actually small, and so the more "structure" of
mv or mp could hamper the iterative convergence. So you could try
converging first with a larger smearing (.04 to .08), and then restart
with the "right" one. I'd be somewhat surprised to see much difference
between gaussian/mp/mp for say 0.07 Ry ~ 1 eV of smearing. Of
course, such a large smearing shoudl be used only to help
get convergence, and then reduced.

				nicola



On 27/04/2012 10:47, Hande Ustunel wrote:
> Dear PWSCF community,
>
> While we are on the subject of convergence, I would like to inquire about
> something that we have consistently been observing in a very large number
> of the systems we have studied (examples being metal atoms on g-Al2O3, CeO2
> surfaces etc). While gaussian smearing consistently produces convergence
> without problems at first shot for a smearing width of 0.01-0.02, we
> frequently have problems with mv smearing (our favorite) for the same
> width. Systematically exploring a giant portion of 6 or 7-dimensional
> parameter space (kpoints, smearing width, vacuum length so forth)
> eventually results in convergence with mv but we usually have to fiddle a
> lot; with gauss instead, anything basically works. Would anyone be able to
> confirm this experience or better still provide some insight on the
> realtive ease of using mv vs gauss?
>
> Thank you very much in advance.
> Best regards,
> Hande Toffoli
>
>
> --
> Hande Toffoli
> Department of Physics
> Office 439
> Middle East Technical University
> Ankara 06531, Turkey
> Tel : +90 312 210 3264
> http://www.physics.metu.edu.tr/~hande
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-- 

----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL