[Pw_forum] Raman calculation using gamma point

[Paolo Giannozzi]

On Thursday 14 April 2005 09:34, Gana Natarajan wrote:

> I am trying to calculate the Raman spectrum using the 2n+1 method
> for a small molecule with only the gamma point for k-point sampling.

see at the end

> There are two problems. First the nscf calculation with 'raman' as the
> keyword assumes there are 7 kpoints even though I specified only the
> gamma point in the input file.

this is not a problem: the Raman calculations needs to calculate
wavefunctions at several k+b vectors around k

>[...]  from phq_readin : error #         1
>       cannot start from pw.x data file using Gamma-point tricks

you cannot specify K_POINTS {Gamma} but 
K_POINTS
 1
 0.0 0.0 0.0  1.0
The difference is that in the first case pw.x uses "Gamma-point tricks"
that are not used in the second case. The phonon code does not 
implements these tricks.

The Raman code is, how to say? not in a good shape right now.
It was written by somebody who then left for a job in real world.
I had serious trouble in extracting any meaningful number from it.
I changed the factor multiplying the Raman tensor and a few
other things (these changes will appear in the new version).
Eventually I gave up and tried a different approach, based on 
second-order response (M. Lazzeri and F. Mauri, Phys. Rev. Lett. 90, 
036401 (2003)). This code is available since a few days in the CVS 
branch (courtesy of Michele Lazzeri) but it is not yet documented.

Paolo 

-- 
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