[QE-users] seeking hints for nscf convergence in supercell

[Giuseppe Mattioli]

Dear Patrizio
Are you sure you need ecutwfc=100 Ry and ecutrho=12*ecutwfc to  
converge US pseudopotentials? Have you performed convergence tests on  
simple systems or flicked through sssp tables (I see 30/240 for  
pslibrary US Mg and 40/320 for gbrv US Sb...)?
However, for your nscf step you require a 45 45 31 k-points mesh. It's  
a lot of k-points. I don't know what is the purpose of your  
calculations, so I can't say if you are uselessly oversampling the  
Brillouin zone. But there is no free lunch...
HTH
Giuseppe

PS maybe we are nearest neighbours, you may phone... :-D

Quoting patrizio.graziosi at cnr.it:

> Hello everybody,
>
> I'm seeking advice to fasten nscf convergence (or make it possible).
>
> I'm using qe 6.8 to study Mg3Sb2 / Mg3Bi2 solid solutions, as first  
> step I'm starting with a Mg3Sb2 2x2x2 supercell (40 atoms) to  
> compare with simple Mg3Sb2 cell (5 atoms).
>
> The calculation track is scf --> nscf --> fs.
>
> For the simple cell it runs smoothly (and I'm confident it is okay  
> as I compared with a colleague of mine relatively expert in DFT, not  
> qe).
>
> For the supercell the scf converges in around 1 day of CPU time, but  
> the nscf does not converge in ~ 12 days of CPU time - say 23 hours  
> with 4 nodes, 6 tasks, 6 cpu-per-task, npool=24, are not sufficient  
> to reach convergence.
>
> I attach the .in files for both scf and nscf, as well as the nscf  
> out file. There are several eigenvalues not converged but I am not  
> sure if this is the cause of the long time required for convergence  
> (I've read in the forum this can be often regarded as a warning).
>
> So, since I have limited computational resources, before making many  
> trial-and-error attempts I would seek hints and advices to  
> understand if it is only a matter of make a more intensive  
> calculation (I've read the restart mode is not recommended for nscf  
> so I should ask for more nodes), or to improve the input file in an  
> aware way.
>
>
> Sorry if this is a time waste for you, and thanks in advance for any support,
>
> Patrizio
>
> -- 
>
> Patrizio Graziosi, PhD
>
> Research Scientist
>
> CNR - ISMN
> Institute for the Study of Nanostructured Materials



GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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E-mail: <giuseppe.mattioli at ism.cnr.it>