[Pw_forum] phonon calculation of alkaline metals

[Aritz Leonardo]

Muchas Gracias Jose!!

Tu e-mail ha sido una bendicion, ahora me toca hacer pruebas con la 
supercelda que me has dicho.

MUCHAS GRACIAS!

Jose C. Conesa wrote:

>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
>
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>  
>

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