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["张�s" <zhangting1980323@gmail.com>]

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

2008/6/29 <mjv500@york.ac.uk>:

Hello,

it sounds like you are only calculating the q=0 phonons. I don't know where you are extracting the gkk from, but you normally need many more phonon qpoints. The anaddb code will interpolate for q which are not in your default set. If you have too few qpoints you can get bad interpolation and the phonon linewidth can go negative artificially. Check if any of your phonons have gone soft (w<0) - this could also mess up the gkk and linewidths.

On a more general note, the gkk matrix elements don't absolutely need to have a particular sign; the observable quantity is the linewidth, which is related to the gkk squared. There are, however, hand waving arguments for the gkk being positive in stable systems.

Concerning the wires, most monoatomic straight wires are unstable wrt deformation into a zigag (see Phys. Rev. B 74 , 153408 (2006) and the references to Batra and others therein), so this could be the unstable mode you are seeing in a 2 atom unit cell.

Matthieu