[Wien] How to calculate optical absorption edge ?

[Rocquefelte]

If you plot epsilon2 (the imaginary part of the dielectric function) and 
use the tangent method, you can estimate the absorption edge from WIEN2K.
However, it requires to use a proper functional, allowing to overcome 
the inherent problems of DFT, which is a ground state theory, i.e. LDA+U 
for correlated systems or hybrid functional...

Regards

Xavier

Le 7/9/2013 7:00 AM, Masood Yousaf a écrit :
> Dear Sir, thanks for your elaboration. Need some further explanation 
> to overcome the confusion. Is it possible to determine accurately 
> absorption edge from WIEN2k code only i.e., from case.epsilon ?
>
> Best wishes
> Masood
>
>     ------------------------------------------------------------------------
>     *From:* Rocquefelte <xavier.rocquefelte at cnrs-imn.fr>
>     *To:* A Mailing list for WIEN2k users
>     <wien at zeus.theochem.tuwien.ac.at>
>     *Sent:* Tuesday, July 9, 2013 12:40 PM
>     *Subject:* Re: [Wien] How to calculate optical absorption edge ?
>
>     Experimentally an absorption edge (optical gap) is estimated using
>     the tangent method.
>     Thus you should do the same with your calculated data to compare
>     optical gap values, which differ from a fundamental band gap.
>
>     Indeed, a fundamental band gap is the smallest band gap allowing
>     to excite an electron in unoccupied states (deduced from
>     electrical measurements - thus no transition rules).
>     In contrast, the optical gap is related to an excitation using
>     photons, thus it respects the dipolar transitions rules (this gap
>     is usually larger than the fundamental band gap).
>
>     Regards
>
>     Xavier
>
>
>     Le 7/9/2013 3:07 AM, Masood Yousaf a écrit :
>>     Dear wien2k users
>>
>>     Kindly suggest how to calculate optical absortion edge in wien2k
>>     ? How it differs from band gap of material ? I read that first
>>     critical point in imaginary part of the dielectric function
>>     represents optical absorption edge. But how to ensure that which
>>     is the first critical point. Kindly guide
>>
>>     Best wishes
>>     Masood
>>     Universiti Tecknologi Malaysia
>>
>>
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