The ABINIT Users Mailing List ( CLOSED )

[Pascal Boulet <pascal.boulet@univ-provence.fr>]

Dear Tao,

To have the LO-TO splitting you have to calculate the perturbations w.r.t. the electric field. You can do this in two steps:
first you calculate the derivative of the wavefunction w.r.t. to k (the so-called ddk in ABINIT) with rfelfd=2
second you calculate the response to the electric field with rfelfd=3

This is documented in the file trf2_1.in (sets 2 and 3) of the tutorial (response-function 2)

Last step:  to get a proper plot of the phonon band structure you have to make sure that you put in the file "your_filename.freq" the splitting at the gamma point before issuing the band2eps command. Otherwise you will not see it on your graph! Read carefully section 5 of the response-function 2 of the tutorial. This is explained.

As to the documentation of rfdir, see:
http://www.abinit.org/Infos_v5.5/input_variables/varrf.html#rfdir
In your case you have to set rfdir to 1 1 1.

Hope this help,
Best regards
Pascal

GaoTao wrote:

COL102-W80694F8D21AABFD6B8553BB9760@phx.gbl" type="cite"> Dear Prof Gonze,
 
Thanks very much for your explanantion.
 
when I use 'rfdir=1 1 1' to compute the phonon response of all direction, I did no get any LO-TO splittings of A2u in my Tetragonal anatase TiO2.  And more there are no details about the descriptions for the usage of 'rfdir' in mannual. Would you please let me know how I can set the parameter 'rfdir' in my input file or some other  parameters in order to get LO-TO splittings for A2u in my Tetragonal anatase TiO2? 

Acutally, I have tried different way to approach the GAMMA point in BZ space, but I still didnot get the A2u LO-TO splittings in my pnonon band structure. I am eager to know what is the reason for this.
 
Regards,
 
Tao
 
> From: xavier.gonze@uclouvain.be
> To: forum@abinit.org
> Date: Mon, 13 Apr 2009 21:58:50 +0200
> Subject: Re: [abinit-forum] the LO-TO splitting for the IR active A2u in TiO2 and SiO2
>
> Dear T Gao,
>
> On 13 Apr 2009, at 21:46, gaotaoscu@hotmail.com wrote:
>
> > Hello everybody,
> >
> > When read the paper 'Lattice dynamics and dielectric properties of
> > TiO2
> > anatase: A first-principles study' published PHYSICAL REVIEW B 66,
> > 155213
> > (2002). there is one point I can not undserstand, i.e., why there
> > is a LO-TO
> > splitting for the IR active A2u?
> >
> > To my best of knowledge, A2u is only a one dimmensional
> > representation for
> > group D4h(4/mmm). It is quiet easy to find the LO-TO splitting for Eu
> > representation in ABINIT calculation, because it is a two dimmensional
> > representation. so it could have the LO-TO splitting. And the same A2u
> > Splitting also exists in article 'Lattice dynamics and Dielectric
> > Properties of
> > SiO2 Stishovite' published on PRL, vo72, p1686,1994.Why there is a
> > LO-TO
> > splitting for the IR active one dimmenstional representation A2u
> > mode? Could
> > it be splitted in optical measurement?
>
>
> The frequency of this A2u (at Gamma q=0 in principle) will depend on
> the way the limit q->0 is taken.
> Indeed, there is a non-analytic contribution to the dynamical matrix,
> and, for different
> branches of the phonon dispersion curves, the limit of the A2u mode
> frequency at Gamma will be different.
> So, along some directions, the electric field will not couple (TO),
> while for one direction,
> there will be a full coupling to the electric field (LO). This can be
> seen in neutron experiments.
> For the equations, see section VIII B of Phys. Rev. B 55, 10355 (1997).
>
> Best regards,
> Xavier

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-- 
Dr. Pascal Boulet, Computational Chemist
University Aix-Marseille I
Laboratoire Chimie Provence, UMR6264
Centre Saint-Jerome, Bat. MADIREL
F-13397 MARSEILLE Cedex 20, France
Tel. +33 (0) 491 63 71 17 Fax. +33 (0) 491 63 71 11
courriel: pascal.boulet@univ-provence.fr
http://www.lc-provence.fr
http://allos.up.univ-mrs.fr/boulet