[Wien] Optic

[Lawal Mohammed]

Dear Dr. Alias,
Thank you very much for the explanation.
Please what really cause these excitations since we're dealing with the ground state properties?
 Any comment in this regard is highly appreciated.
Lawal Mohammed
 
     From: Elias Assmann <elias.assmann at gmail.com>
 To: A Mailing list for WIEN2k users <wien at zeus.theochem.tuwien.ac.at> 
 Sent: Friday, September 18, 2015 6:56 PM
 Subject: Re: [Wien] Optic
   
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On 09/15/2015 08:36 PM, Lawal Mohammed wrote:
> I understand that in gga/lda the Kohn-Sham eigenstates are regarded
> as excited only when the scissors operator is non zero, is this
> correct? If yes then how valid is optical calculation when the
> scissors operator is set to zero? If no then what bring about the
> optical excitations that allow determination of some optical
> properties?

The “scissors operator” is simply a rigid shift of the unoccupied
bands in energy (where rigid means that the states themselves are not
changed).  This is done to “correct” band gaps which come out wrong
(usually too small) in DFT.  An optical excitation takes an electron
from an occupied to an unoccupied state.  This means that the scissors
operator will change the energies where these excitations are
possible, e.g. to adjust the optical gap in your calculation to the
experimental value.

HTH,

    Elias


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