[Pw_forum] Acoustic branches

[merlin.meheut at impmc.jussieu.fr]

Dear Stefano,

My point is to compute the so-called "acoustic contribution" to  
thermodynamical
properties such as heat capacity or so. It is commonly used in  
empirical models
calculations (such as Debye-Einstein models). Moreover, I am working  
with big structures, and I have some trouble obtaining good dispersion  
curves near Gamma, so I would like to extract information from the  
acoustic branches over the complete Brillouin zone. This is related to  
the point of calculating thermodynamic properties with only gamma  
frequencies, so that a model has to be set for the acoustic part (  
because, as it goes to zero, it can create some singularities). I am  
not sure to be very clear myself, this is exploratory.
To say it in a different way, experimentally we have information on  
optical frequencies (at gamma point) but not easily on what's below  
(except sound wave velocity but as I said before I have some trouble  
having a good curve at low q for acoustic modes ). The goal is to  
calculate the contribution of those frequencies on which we have no  
information by optical spectroscopy.

Thanks in any case for your idea of "following the eigenvectors", this  
is probably the better one. Is there an easy way to do it? (I think I  
know the answer).

Best wishes,

Merlin




Quoting degironc <degironc at sissa.it>:

> close to gamma the acoustic branches have linear dispersion.
> Away from gamma one should follow the modes that are connected
> to these three modes at gamma. If there is no symmetry these are just
> the three lowest branches. If there is some symmetry they can cross
> a few branches with different symmetry labels and should be possible
> to follow the acoustic ones comparing the eigenvectors.
> But I dont see any special meaning of acoustic modes far away from
> gamma.
> What are you looking for ?
>
> stefano
>
> meheut at impmc.jussieu.fr wrote:
>
>> Dear Drs,
>>
>> I would like to use pwscf to compute the acoustic branches only of the
>> crystal. I already computed the complete phonon dispersion, but the
>> acoustic branches are crossing with other modes. Do you think possible
>> to disjoin the acoustic modes frequencies from the other modes?
>>
>> Best wishes.
>>
>> Merlin meheut
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