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["Xiulin Ruan" <ruan@purdue.edu>]

Dear Nuno,

Thanks for your inspiring discussions, see my comments below.


> -----Original Message-----
> From: Nuno A. G. Bandeira [mailto:nuno.bandeira@ist.utl.pt]
> Sent: Thursday, June 21, 2007 9:08 AM
> To: forum@abinit.org
> Subject: Re: [abinit-forum] TDDFT for periodic system
> 
> Xiulin Ruan wrote:
> > Thanks for your comments right to the point. The TDDFT calculations on
> Ti3+
> > doped finite clusters do tell some physics and predict energy gaps well,
> but
> > they are not good enough when we ask tougher questions. For example, how
> > would the energy levels (and optical properties) of Ti3+ be affected by
> > different host, like Al2O3, Y2O3, YAG, etc? I assume the cluster
> > calculations are incapable since they are not able to reproduce the
> crystal
> > fields exactly.
> 
> There are many articles being published nowadays in the field of
> chemistry and chemical physics with finite cluster models. It's a fair
> approximation that's nothing to be ashamed of.
> 
> You can't reproduce the crystal medium "exactly" but then the functional
> you use and the basis set is also not exact. You have to start somewhere.
> 
> In your case why do you assume the cluster calculations won't manage it
> ? In principle the bigger the cluster the better the result.
> The crystal field transitions should be influenced by the acidic nature
> of the host trication which will condition how oxygen binds to the Ti3+
> centre. So you should always get different results for different host
> crystals whether you use clusters or PBC.
> 

We have done the cluster calculations and we like the results. Here we are
just trying to find out the possibilities of improving our calculations. As
you said, the effects of different hosts on the energy levels can be
addressed by using a larger cluster, by filling the first coordination
sphere of the TiO6 octahedron with Al or Y atoms. But we will still have a
problem in studying the electron-phonon coupling in the nonradiative decay
process. We did it using the cluster model and the work was accepted for
publication, but the criticism from a referee still remains that we were
dealing with molecular vibrations, not really lattice phonons. In fact we
are not so satisfied ourselves to simulate the electron-vibration coupling
of the Ti doped cluster but claim it to be the electron-phonon coupling of
Ti doped solids. 


> > Also, we can use a minimum unit cell with one Ti dopant. Although this
> means
> > an unreasonably high doping concentration, but it is still acceptable as
> a
> > first attempt, better than the cluster model I guess. Would this reduce
> the
> > computation time significantly?
> 
> Obviously but is it worth the effort ? You could receive some criticism
> from a referee saying that this model may not be an "exact" depiction of
> the real doping since it's so highly concentrated.
> 
> So if you fill the first coordination sphere of the TiO6 octahedron with
> Al atoms as a first approximation it would still be ridiculous to claim
> that Ti is an impurity.

In view of the concentration, wouldn't the cluster model have a sort of
"higher concentration" since it has a smaller size?

> 
> Cheers,
> 
> --
> Nuno A. G. Bandeira, AMRSC
> Graduate researcher and molecular sculptor
> Inorganic and Theoretical Chemistry Group,
> Faculty of Science
> University of Lisbon - C8 building, Campo Grande,
> 1749-016 Lisbon,Portugal
> http://cqb.fc.ul.pt/intheochem/nuno.html
> Doctoral student @ IST,Lisbon
> --