[Pw_forum] InfraRed intensities

[Stefano Baroni]

On Oct 31, 2006, at 7:15 PM, Manish Jain wrote:

> Hello
>
> I am interested in calculating IR spectra for an insulator. Is the  
> following procedure for calculating IR spectra (intensities and  
> position of peaks) correct?
>
> a) Calculate the scf charge density and wavefunctions at a  
> minimized energy structure.

yep!

> 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?

> c) Use dynmat.x to compute the cross sections of individual modes.

yeap, but only for those wavevectors (q") that are allowed by  
conservation laws (see point b)

> d) Then combine them all using some means to get an IR spectra???

OK, but what means are "some means"?

> Any help is appreciated.

Hope this helps a bit.

Stefano B.

>
> Thanks,
> Manish
>
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
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