[Pw_forum] My phonon dispersion curves are jagged too

[Serge Nakhmanson]

Hi Nicola,

Thanks for your comment. Actually, following Davide Ceresoli's
advice, I always use "cg" as my minimization algorithm. I just
redid my calculation with startingwfc = 'random' instead of
'atomic' and with conv_thr = 1.e-12 instead of 1.e-10. The
results of the new calculation are identical to the "old" one
(i.e., the one that was used to compute phonons in my example).
Anyway, I have other phonon calculations where the symmetry is
reduced from Pm-3m to, e.g., P4/mmm (not sure, though, that this
symmetry is low enough to avoid the problem you mention below),
and the dispersion-curve breaks are present there too.


THX,

Serge

Nicola Marzari wrote:
> 
> Dear Serge,
> 
> 
> in the past we encountered a problem in the cubic perovskites,
> in which the davidson algorithm would converge the top band
> at some k-points to an "excited state". This is, afaik, a problem
> with some algorithms (much more so with diis); a simple
> solution was to use "atomic+random" as a starting point, and maybe
> use "cg" as the minimization strategy in the scf calculation,
> with a very tight tolerance.
> 
> Try that, maybe without redoing the phonons, just to see if the
> calculations converge to the same ground state, or to a lower one.
> 
> 					nicola
> 
> 
> 
> 
> Serge Nakhmanson wrote:
>> Dear All,
>>
>> I am learning how to compute phonon dispersion curves across the BZ and
>> right now I am trying to reproduce the results of a simple calculation
>> for cubic perovskites that Philippe, Karin and company have done a long
>> time ago [PRB 60, 836 (1999)].
>>
-- 
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  Serge M. Nakhmanson               phone: (630) 252-5205
  Assistant Scientist                 fax: (630) 252-4798
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