The ABINIT Users Mailing List ( CLOSED )

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

Dear Nuno,

On 21 Jun 2007, at 15:08, Nuno A. G. Bandeira wrote:

> 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.


Precisely not ! If you make the cluster size tend to the macroscopic  
limit, you will recover the Kohn-Sham band edges
from TDDFT, suppressing all good things that TDDFT do with respect to  
DFT for small clusters.
So, you might stop at the cluster size that gives an agreement with  
experiment, but this is clearly ad hoc.

Xavier