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[GaoTao <gaotaoscu@hotmail.com>]

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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