[Pw_forum] Separation between acoustic modes and optical modes

[stefano de gironcoli]

Dear Umesh
    I'm not sure I understand your problem.
    if acoustic modes and optical ones have different symmetries (LA and 
TO for instance in specific directins) they can overlap in frequency but 
still be distinct objects that you can identify by examining the 
eigenvectors. If they have the same symmetry (typically no symmetry at 
all for a generic q direction in the BZ) they interact and get mixed so 
there aren't purely acoustic or purely optical modes any more.

stefano


On 01/08/2015 13:21, Umesh Roy wrote:
> Dear All,
>            I have calculated modes of vibration of Iron in different 
> directions for a fix phonon wave vector q (in a large number of 
> directions). Since in Quantum espresso phonon modes are printed in 
> increasing order for a given q vector. For a small group of q the 
> acoustic and optical branches do not overlap so, we can easily 
> separate out the acoustic branch from the optical branch. As we 
> increase the magnitude of q the acoustic branch and the optical branch 
> get mixed up. So , how to separate out the acoustic modes or is there 
> any way of filtering the optical modes?
>
>
> Thank you in advance
>
>
> /*Umesh Chandra Roy*
> Research Scholar, School of Physical Sciences
> Jawaharlal Nehru University, New Delhi-110067,
> /
> /India.
> /
> /Email:*umesh24crp at gmail.com* <mailto:Email%3Aumesh24crp at gmail.com>
> /
> /Mobile:*+919868022722*//*
> */
>
>
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