[Pw_forum] InfraRed intensities

[Manish Jain]

Dear Prof. Baroni,

Thanks for your quick response.

>>b) Do a phonon calculation on a grid of q points with epsil set to  true.
>
>wait! phonon absorption is a process in which the initial state is a  
>crystal in the ground state (q=0) and a photon with a momentum q'<<2 \pi /a 
>(let's say, q'\approx 0), and the final state is the crystal  with a phonon 
>with wavevector q" and no photon. Given  momentum  conservation, can you 
>tell which values of the phonon wavevectors  (q") are allowed?

I misstated. I was thinking about calculating the IR spectra AND the phonon 
dispersions.. (the sampling was not meant for the purpose of IR 
intensities). I understand that for IR spectra only the gamma point 
calculation is needed.

>>d) Then combine them all using some means to get an IR spectra???
>
>OK, but what means are "some means"?

By "some means" I was referring to their calculation of the absorption from 
each IR active mode (calculated at the gamma point) and then just adding the 
spectra up to get the complete spectra. I was not sure if the code - 
dynmat.x - did this automatically or if I needed to do this separately.

>
>>Any help is appreciated.
>
>Hope this helps a bit.

This clarifies a great deal. Thank you once again.

Regards,
Manish

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