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[mjv500@york.ac.uk]

Hello again,

ok here we go: what you read from the gkk file is what abinit calculates 
raw, which is "gkk" matrix elements, but not for responses not to phonon 
modes but to single perturbations of 1 atom in 1 direction. The hand waving 
argument is that if you start from an equilibrium structure, the 
deformation potential you are sandwiching to get gkk should be somehow 
positive as you should be increasing the energy. I don't have an article 
for you - that would contradict the meaning of hand waving. However,

(1) In your case I'm still not sure that the structure is mechanically 
stable (not just wrt to phonons but wrt strains as well)

(2) what you are examining in the gkk file are full matrices btw all bands 
in the simulation. Even if my hand waving convinces you that the operator 
should be positive, that has no bearing on it's matrix elements in a random 
basis.

(3) I'm still not convinced of my hand waving because in the definition of 
the gkk there is a scalar product with the phonon displacement vector d. In 
many systems (and at least in the harmonic approximation) d and -d are both 
valid displacement vectors, and this makes the sign of d's scalar product 
with the raw gkk part have an arbitrary sign (the atomic perturbations I 
described above are a specific non-diagonal choice of basis vectors for the 
displacements). Even worse, if you have degenerate modes then you can make 
any rotation of the phonon eigenvectors corresponding to those modes and 
still get the same ep coupling.

anyhow, I wouldn't worry about the sign and instead look at the derived 
physical quantities you are interested in.

good luck

Matthieu

On Jun 29 2008, [GB2312] ÕŜs wrote:

> Dear Matthieu:
>
> Thank you for your kindly reply. My gkk element value are read directly 
> from gkk file, which are extracted from 1WF files with mrggkk code. So 
> the gkk elements I mentioned are directly from abinis.
>
> So you mean that the |gkk|^2 is physical meaningful. I see. I'll read paper
> you mentioned, And can you show me some paper that can prove the gkk should
> be positive in stable systems? Thanks
>
>
> Regards
>
>
> Zhang Ting
>
> Peking Univ
>
> June 30th, 2008