The ABINIT Users Mailing List ( CLOSED )

[Xavier Gonze <gonze@pcpm.ucl.ac.be>]

Dear Bohdan,

Could you try to use "optics" also (see Test_v4#57) ?

Xavier


Bohdan Andriyevsky wrote:
> Dear Konstantin,
> Thank you very much for your reply and advices.
> Because the spectrum epsilon''(E) of my crystal calculated agrees well with
> experimental data (band gap Eg and positions of maxima), there is no need to
> use "scissor" operator. In the DFT-based calculations the calculated Eg is
> often smaller than the experimental one. Therefore, the epsilon'(E=0)
> calculated is often greater than the experimental one. But in the case of my
> crystal, the epsilon'(E=0) calculated using epsilon''(E) and
> Kramers-Kronig's relation (1.4) is much more smaller than the experimental
> one an the same value obtained from the RF calculation using the 'abinit'
> (2.5). So I have obtained two different magnitudes of epsilon'(E=0) (1.4 and
> 2.5 ) using 'abinis'+'conducti'+Kramers-Kronig relation from one hand, and
> using 'abinis' with RF-calculations form the other hand. Therefore, I
> believe, that one of the possible reason of this discrepancy can be a small
> bag in the 'conducti' program: the correct values of epsilon''(E) must be
> 'pi' (3.1416) greater than the present one. Of course, I plan to verify this
> with other crystals.
>
> Regards,
> B. Andriyevsky
> ========================================================
> DSc. Bohdan Andriyevsky
> Technical University of Koszalin
> 2 Sniadeckich Street, 75-453, Koszalin
> Poland
> tel.: +48 94 347 8690, +48 508 070 029
> fax: +48 94 343 3479
>
> ----- Original Message -----
> From: "Konstantin Rushchanskii"
> <Konstantin.Rushchanskii@physik.uni-regensburg.de>
> To: <forum@abinit.org>
> Sent: Tuesday, April 05, 2005 11:46 AM
> Subject: Re: [abinit-forum] So small conductivity
>
>
>
> > Dear Bohdan,
> >
> > I think, there are two reasons for your problem:
> >
> > 1. Be sure your results are well converged with respect to k-points mesh
> > and Ecuts
> > 2. When you use LDA for the band structure investigations, be carefull,
> > because LDA typically underestimates the band gap. For ionic crystals
> > the underestimation is about 2/3 of experimental value, but for covalent
> > crystals with strong anisotropy and with the presence of Wan der Vaals
> > interaction and/or hydrogen bonding (I suppose this all is present in
> > your crystal), the underestimation of the LDA results are not
> > predictible. You have at least two possibility to treat this
>
> incostistance:
>
> >    a. If you want a purely ab initio results, you can obtain the band
> > structure in GW approach. ABINIT can do this, but (as far as I know)
> > only for centrosymmetrical crystals. But for your crystal it is
> > computationally VERY COMPLICATED procedure.
> >    b. If you have some experimental data about Eg, you can apply so
> > called "scissors" operator (rigid shift of all conduction bands with
> > respect to experimentally observed Eg). This correction is very usefull
> > for dielectric properties of the crystals. In ABINIT you can apply this
> > correction by "sciss" input variable.
> >        Any way, you cannot expect very good coincidense between
> > experimental and theoretical results for these quantities.
> >
> > I hope, this helps.
> >
> > Sincerely,
> > K.Rushchanskii
> >
> >
> > > Dear Abinit Users,
> > > When I have calculated the conductivity spectrum g(E) of my crystal (Eg >
>
> 5 eV) using the 'abinis' and 'conducti' exe-files, I have calculated the
> imaginary part of the dielectric permittivity epsilon'' using the relation:
>
> > > g=2*pi*f*epsilon0*epsilon'' , where f=E/h (h=6.62E-34 is Planck
>
> constant), and epsilon0=8.85E-12.
>
> > > The structure of the spectrum epsilon''(E) obtained agrees well with the
>
> corresponding experimental data. Then I have calculated the real part of the
> dielectric permittivity epsilon'(E) using the known Kramers-Kronig relation:
>
> > > epsilon'(E)-1=(2/pi)*integral{[t*epsilon''(t)*dt]/[t^2-E^2]}.
> > > The magnitude of the epsilon'(0)=1.4 obtained by this calculation is much
>
> smaller than the experimental one and the analogous magnitude obtained from
> the RF calculations (2.5) using the Abinit.
>
> > > Maybe anyone already has done similar analysis? I will be thankful for
>
> any advices to resolve my problem.
>
> > > Regards,
> > > Bohdan Andriyevsky
> >
> >
> > --
> > K.Rushchanskii, Dr.
> > Theoretische Physik, Universität Regensburg
> > 93040 Regensburg Germany
> > tel: +49 (0) 941-943-2466  fax: +49 (0) 941-943-4382