[Pw_forum] How to calculate real space inter-atomic force constants excluding the contribution due to macroscopic electric field

[Eyvaz Isaev]

Dear Ravi,

>From INPUT_PH

epsil (default=.false.) - if .true. in a q=0 calculation for a non metal the macroscopic dielectric constant of the system is computed. Do not set epsil to .true. if you have a metallic system or q/=0: the code will complain and stop

So, if you like to exclude the electric field contribution in semiconductors (insulators) just consider them as a metallic system. But later you will find an inconsistency between phonons you calculated and experimental data as the electric field leads to splitting of LO-TO modes in semiconductors. 

> I am new to pwscf
Welcome!

Bests,
Eyvaz.

-------------------------------------------------------------------
Prof. Eyvaz Isaev, 
Theoretical Physics Department, Moscow State Institute of Steel & Alloys, Russia, 
Department of Physics, Chemistry, and Biology (IFM), Linkoping University, Sweden 
Condensed Matter Theory Group, Uppsala University, Sweden 
Eyvaz.Isaev at fysik.uu.se, isaev at ifm.liu.se, eyvaz_isaev at yahoo.com


--- On Wed, 9/3/08, Ravi Maranganti <ravitri2k at gmail.com> wrote:

> From: Ravi Maranganti <ravitri2k at gmail.com>
> Subject: [Pw_forum] How to calculate real space inter-atomic force constants excluding the contribution due to macroscopic electric field
> To: pw_forum at pwscf.org
> Date: Wednesday, September 3, 2008, 10:05 PM
> Hello All
>    How should i calculate the real space interatomic force
> constants with
> the contribution due to the macroscopic electric field
> being excluded.
> According to Giannozzi et al (Ab initio calculation of
> phonons in
> semiconductors, Phys Rev B, v. 43, p.7231), the
> contribution to the
> dynamical matrix in polar materials can be written as a
> combination of an
> analytical term and a non-analytical term (at low
> k-vectors). The
> non-analytical term involves the Born effective charges and
> the dielectric
> constant. so, should i subtract this term from the
> dynamical matrix at all
> k-vectors (or only at certain k-vectors?) and then use
> q2r.x to obtain the
> real space force constants? Also, should i *not* specify
> .epsil. = true?  I
> am new to pwscf so please bear with me...thanks!
> 
> Ravi Maranganti,
> PhD Candidate, Mechanical Engg Dept,
> Univ of Houston, TX
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