[Pw_forum] Phonon DOS in noncubic materials

[Stefano Baroni]

I think (others may want to confirm or deny) that this depends of the  
non-analytic behavior of the dynamical matrix in the long-wavelength  
limit. Because of this, phonon frequencies at q=0 are strictly  
speaking not defined. Their limit as q->0 is indeed well defined only  
once the direction of q and of the phonon polarization is specified.  
For cubic materials, this non-analiticity gives rise to the well known  
LO-TO splitting, whereas for crystals of lower symmetry the situation  
is more complicated. If the DOS is calculated using a very fine mesh  
of q points, I would expect that the result would be independent on  
what happens around q=0. If, instead, too few points are used, I would  
not be surprised if the above-mentioned non-analytic behavior would  
severly undermine the accuracy of any sensible interpolation procedure  
needed for any integration (such as the one used for DOS  
calculations). Hope this is not too wrong, and of some help as well. SB

On May 23, 2008, at 7:25 AM, rprasad at iitk.ac.in wrote:

> Dear PWSCF users and developer,
>
> There is a comment in matdyn.f90 that it may not work for noncubic
> materials for calculating phonon DOS. Why is so? Is there a simple  
> way to
> modify the program for noncubic materials?
>
> R. Prasad
> IIT Kanpur
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> Pw_forum at pwscf.org
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
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