[Pw_forum] frequencies using finite differences instead of perturbation theory?

[Paolo Giannozzi]

On Jan 17, 2011, at 14:06 , Martin Andersson wrote:

> I am looking into the effects of empirical dispersion, and as far as I
> can see from the phonon output file, the effect from dispersion isn't
> included when the frequencies are calculated using ph.x. Is there a  
> way
> to use finite differences in the QE suite as opposed to perturbation
> theory in order to get frequencies with empirical dispersion included?

there is for sure more than one way, but none currently implemented and
ready to use, AFAIK. Once upon a time there used to be a frozen-phonon
code, but nobody was maintaining it so it has been removed.
In the Gamma/ subdirectory there is a piece of code that evaluates Raman
coefficients using finite differences on the dielectic tensors. It  
was written
for testing purposes several years ago and could be easily adapted to
frozen-phonon calculations, but it will need some work.

P.
---
Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222