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["Nuno A. G. Bandeira" <nuno.bandeira@ist.utl.pt>]

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.

> 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.

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
--