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Dear Joey Davis,<br>
<br>
Your question is curious in some points because it contains a part of
the answer (so it looks like a discussion more than a question). <br>
However, I can try to give you my point of view on this topic. As soon
as we are using an adjustable parameter (scissor operator for the band
gap) no prediction can be done on the quantity we are correcting.
However, by modifying this gap value with a scissor operator,
prediction of related quantity (dielectric function, refractive index
...) can be done. <br>
So in my point of view, the comparison of DFT+SO (SO: Scissors
Operator) optical data with the experiment is definitely not a proper
way to define the experimental band gap. <br>
The experimental band gap must be determined from experiments only (or
appropriate treatment of the so-obtained data). <br>
<br>
Another important point is the difference between the fundamental band
gap and the optical band gap. Depending on the experiments you are
looking you will access to a different band gap (electronic or optical
gap). <br>
<br>
If you really wants to do optical band gap prediction, you should look
at method that goes beyond DFT (GW, hybrid approaches, exact exchange,
...). However, I could insist on one last point, in the different
systems I have treated I have observed that usually the scissor
operator we are using is nearly the same (or the same) for a given
chemical formula. Then one possibility is to predict the gap variation
more than the absolute gap value as a function of structure variation.
More generally, DFT alone does not allow to predict the absolute value
of the band gap but relative data can be obtained in some cases. <br>
<br>
With my Best Regards<br>
<br>
Xavier Rocquefelte<br>
<br>
<br>
<br>
Joey Davis a écrit :
<blockquote
cite="mid20070718024335.32239.qmail@web38203.mail.mud.yahoo.com"
type="cite">
<div class="MsoNormal" style="margin: 0in 0in 0pt;"><font
face="Times New Roman" size="3">Dear WIEN users:</font></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;"><o:p><font
face="Times New Roman" size="3"> </font></o:p></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;"><font size="3"><font
face="Times New Roman"><span style=""> </span>I am trying to
calculate the optical and electronic properties of a compound. I only
have the structure and optical conductivity spectra got from
experiment. </font></font></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-indent: 12pt;"><o:p><font
face="Times New Roman" size="3"> </font></o:p></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-indent: 12pt;"><font
face="Times New Roman" size="3">You know that the DFT cannot give the
accurate band gap. Scissors operator is<span style=""> </span>introduced,
that is, the conduction bands are shifted. Then the spectra can be
similar to the experiment.</font></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-indent: 12pt;"><o:p><font
face="Times New Roman" size="3"> </font></o:p></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-indent: 12pt;"><font
face="Times New Roman" size="3">Now, we got the band gap (Eg<sub>1</sub>)
from calculation. With scissors operator (Eg<sub>2</sub>), the shape of
the mainly peaks in the optical conductivity spectra agrees well with
the spectra of experiment. Can we declare the real band gap (the band
gap from experiment) is equal to Eg<sub>1</sub> +Eg<sub>2</sub>? <span
style=""> </span>Can we regard that this is a method to get the band
gap in our compound? </font></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-indent: 12pt;"><o:p><font
face="Times New Roman" size="3"> </font></o:p></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-indent: 12pt;"><font
face="Times New Roman" size="3">Can anyone tell me, or give me some
references about the question?</font></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-indent: 12pt;"><o:p><font
face="Times New Roman" size="3"> </font></o:p></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-indent: 12pt;"><font
face="Times New Roman" size="3">Many thanks!</font></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-indent: 12pt;"><o:p><font
face="Times New Roman" size="3"> </font></o:p></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-indent: 12pt;"><font
face="Times New Roman" size="3">Joey Davis</font></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-indent: 12pt;"><o:p><font
face="Times New Roman" size="3"> </font></o:p></div>
<p> </p>
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</blockquote>
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