[Pw_forum] phonon calculation of alkaline metals

[Jose C. Conesa]

Dear colleague,
On 27 Apr 2005 at 16:25, Aritz Leonardo wrote:
> Hello all!
> 
> I am calculating the phonon dispersion curves of a monolayer of Li (hcp).
> Crystal energies and electronic band structure of my calculation are in good agreement with 
> literature but, when I calculate phonon dispersion curves something strange happens. 
> 
> I first obtain ph frequencies in symmetry points ( gamma, K and M) and all of them seem to be 
> alright except for the first branches at K and M which are negative. 
> ....
> I don't understand at all what can be going on. 
> Thanks a lot!
> 
As is well known, a negative frequency at gamma (assuming that the 
forces at this geometry are zero) means that there is a normal 
displacement that leads to energy decrease, i.e. the system is unstable 
against a decrease of symmetry.
So then what can be the meaning of a negative frequency at a non-
gamma k point? If you define a supercell (e.g. with double value of a, b 
and/or c), the inverse space folds and that k point may become 
gamma. Then: a negative frequency means that with the multiplied cell 
a breaking of symmetry which is not possible with the original cell 
decrases the energy. In other words: the system is unstable against a 
distortion which leads to a periodicity superstructure. You should look 
for a cell doubling (or tripling...) which brings your negative frequency 
into the gamma point, identify the corresponding normal mode, distort 
the structure along that mode and relax it. In some cases this must be 
repeated. Ultimately all negative frequencies disappear and you find a 
system with a different (i.e. multiplied) periodicity and larger primitive 
unit cell. In a surface (your system) this is called a reconstruction (a 
very typical thing).
 Whether this effect is truly physical or an artifact due to bad 
pseudopotential, etc. is a different question.
Hope this helps,
Jose C. Conesa
Instituto de Catalisis y Petroleoquimica, CSIC
Campus de Cantoblanco
28049 Madrid  -  Spain
Phone Nr. 34-91-5854766
Fax Nr. 34-91-5854760