[Pw_forum] about phonon by grid

Marcel Mohr marcel at physik.tu-berlin.de
Tue Sep 18 15:02:59 CEST 2007


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
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