# [Pw_forum] Re: Calculating generalized susceptibility with PWscf

stewart at cnf.cornell.edu stewart at cnf.cornell.edu
Wed Sep 27 22:20:33 CEST 2006

Hi Stefano,

The generalized susceptiblity I'm interested in appears in discussions of
nesting vectors in Fermi surfaces.  It can be expressed in rough latex
format as

Chi(q) = \Sum_{ij,k} \frac{f(\eta_{k,i})[1-f(\eta_{k+q,j})]} {\eta_{k+q,j -
\eta_{k,i}}

where f is the Fermi function, \eta is the energy for the given band and k
point, i and j are band indices.  Only bands that cross the Fermi energy are
included in the sum.  I can send you a pdf file with the equation if the
above is still unclear.

I am putting together a small code to calculate this based on PWscf results,
but I wanted to make sure that this wasn't available as a feature of PWscf.
No sense in reinventing the wheel...

Thanks,

Derek

Stefano Baroni writes:

>
> On Sep 25, 2006, at 3:11 PM, stewart at cnf.cornell.edu wrote:
>
>> Hi everyone,
>>  I was wondering if it is possible to calculate the generalized
>> susceptibility, chi(q), with the PWscf code.
>
> what is a "generalized susceptipility"?
>
> PHONON can calculate the macroscopic dielectric constant "out of the
> box". With little work one could obtain the G/=0 components of the
> response to an applied electric field (q->0). With some more work, it
> would not be difficult to calculate epsilon^{-1}(q,q) (I understand  that
> my notation may not be more clear than yours ;-)
>
> S.
>
>> If someone could point me to the relevant documentation or  examples, I
>> would greatly appreciate it.
>> Thanks,
>> Derek
>>
>> ################################
>> Derek Stewart, Ph. D.
>> Scientific Computation Associate
>> 250 Duffield Hall
>> Cornell Nanoscale Facility (CNF)
>> Ithaca, NY 14853
>> stewart (at) cnf.cornell.edu
>> (607) 255-2856
>> _______________________________________________
>> Pw_forum mailing list
>> Pw_forum at pwscf.org
>> http://www.democritos.it/mailman/listinfo/pw_forum
>
> ---
> Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -
> Trieste
> [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)
>
> Please, if possible, don't  send me MS Word or PowerPoint attachments
> Why? See:  http://www.gnu.org/philosophy/no-word-attachments.html
>
>
>

################################
Derek Stewart, Ph. D.
Scientific Computation Associate
250 Duffield Hall
Cornell Nanoscale Facility (CNF)
Ithaca, NY 14853
stewart (at) cnf.cornell.edu
(607) 255-2856