[Wien] How to calculate optical absorption edge ?

[Masood Yousaf]

I would be grateful if you guide me how to apply this tangent method to the data of imaginary part of the dielectric function. can I use Origin to apply this ?
 

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 2:30 PM
>Subject: Re: [Wien] How to calculate optical absorption edge ?
> 
>
>
>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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>>
>>
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