[Pw_forum] about phonon by grid

[William Yim]

Hello,

This may happen when the calculations are not converged very well.  In
this case, you may need to check: 1) energy cutoffs for wavefunction and
charge density; 2) k-point settings for your scf calculations; 3)  
convergence threshold. In principle, if you use a denser q-mesh for IFC
calculation, you may also need a denser k-mesh for your SCF. Correct me if
I am wrong.

Best regards,
William

On Tue, 18 Sep 2007, Marcel Mohr wrote:

> Hi
> 
> On Tue, 18 Sep 2007, Ezad Shojaee wrote:
> 
> >
> > Hi when i calculate the IFC's by a (2 2 2) grid, and also (3 3 3) the
> > dispersion curve seems to be reasonable and also converging; but when
> > i try (4 4 4) grid, it comes with a lot of huge negative figures! what
> > may be the problem?
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> 
> Hi Ezad
> 
> 4 4 4 is not so big. I would think small changes will occur if you 
> increase q-grid.
> What system do you calculate?
> Are the acoustic frequencies negative, or other ones too. How did you 
> caluclate the dispersion, did you impose acoustic sum rule or similar?
> Maybe your system is not well relaxed, e.g., you are in a local 
> energy minimum and not the global one?
> 
> Cheers Marcel
> 
> ________________________________________________________________________
> Marcel Mohr			Institut für Festkörperphysik, TU Berlin
> marcel(at)physik.tu-berlin.de	Sekr. PN 5-4
> TEL: +49-30-314 24442		Hardenbergstr. 36
> FAX: +49-30-314 27705		10623 Berlin
> 
> 

-- 
Dr. Wai-Leung Yim
Institut fuer Reine und Angewandte Chemie,
Theoretische Chemie,
Carl von Ossietzky Universtaet Oldenburg,
26129 Oldenburg,
Germany
Email: wlyim at puccini.che.pitt.edu             
Phone:	+49-441-798-3950 (office)              
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