[Pw_forum] Separation between acoustic modes and optical modes

[stefano de gironcoli]

On 03/08/2015 08:01, Umesh Roy wrote:
> Dear Stefano,
>          Yes we can identify the type phonon modes by looking at the 
> eigen vectors whether it is LA or TO. To clarify my question here in 
> the following I attached the input file for matdyn.x to compute the 
> phonon frequencies from interatomic force contant at any orbitrary q 
> vectors. I have taken a fixed q vector of magnitude |q|=0.4 in the 
> units of (2*pi/a) and say a number of directions of 196. I calculated 
> phonon frequencies along all these directions. For small group of q 
> vectors we know that first three modes are acoustic modes and the 
> higher modes are optical modes. As we increase magnitude of q vectors 
> we can have optical modes lower than the acoustic modes in magnitude 
> in a specific direction. As we see that the phonon frequencies are 
> written in the file *fe.freq* increasing order. My question is if they 
> are printed as increasing order then how do we know which frequency 
> belongs to acoustic branch and which frequency belongs to optical branch?
you don't.
or rather define what you mean by acoustic and apply the definition. :)
I can think of at least two definitions and they differ, so it's up to you.
It may be useful to ask yourself WHY you want to separate the acoustic 
branches from the others.

stefano

>
>
>
> Thank you.
>
>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20150803/13cfa0e9/attachment.html>