[Pw_forum] IR Raman intensities

[Paolo Giannozzi]

On Wednesday 16 March 2005 11:29, BOUYER Frédéric 153746 wrote:

> Could you please remember me if pw can calculate IR or Raman
> intensities, during vibrational frequencies calculations ?

for IR, you just need to calculate the response to electric fields at 
q=0 (epsil=.true.). Then you can use program "dynmat.x" to calculate
the cross sections for individual modes.

For Raman, things are less straightforward. The code "phcg.x" may
calculate (nonresonant) Raman coefficients, using the Gamma point 
(k=0) only and no ultrasoft PP, and finite differences (very slow for
systems with many atoms). Again, program "dynmat.x" calculates the 
cross sections for individual modes, but there are a few problems that 
will be corrected soon.

The code "ram.x" calculates Raman coefficients (no US-PP) using the 
(2n+1) problem, but it is also very slow for large systems and I am not 
sure that the units are correct.

There is a different code, using second-order response (no US-PP), in the 
CVS branch, but instructions on how to use it are not yet available.

If you want to use US-PP, the only option is to use the phonon code and 
to perform derivatives of \epsilon with finite differences, one by one. 

P.

-- 
Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
Scuola Normale Superiore    Phone:   +39/050-509876, Fax:-563513 
Piazza dei Cavalieri 7      I-56126 Pisa, Italy