[Wien] case.inkram energy shift

[Gavin Abo]

> I am now doing optical properties calculation for semimetals. I have a 
> few question related to the energy shift in case.inkram file. If I 
> don't do energy shift, I can't get a reasonable results compared to 
> experiments. That brings me into two questions.
>
>
> First, what is energy shift here?

It is a correction to the DFT band gap. Refer to:

http://www.wien2k.at/events/ws2006/Optics_Vienna_April_2006.pdf (slide 16)
http://etsf.polytechnique.fr/sites/default/files/dft-II-silva.pdf (slide 8)
http://www.mail-archive.com/wien%40zeus.theochem.tuwien.ac.at/msg13017.html

> How do you determine whether you need energy shift or not?

Compare the calculated and experimental results.  If they match, you 
don't need it.

> Second,how do you determine the exact number of shift, is it shown in 
> some file?

There are two methods used to apply the scissors operator (energy 
shift): the first is to set it so that the DFT band gap matches with the 
measured band gap before the optical calculation and the second is two 
perform the optical calculation followed by adjusting it in subsequent 
calculations until the calculated result (major peak) aligns with the 
measured one.  The later approach is usually used. From page 196 in:

/Density Functional Theory of Molecules, Clusters, and Solids/, 2nd Ed., 
D. E. Ellis, Ed., Springer Netherlands, 1996: 
http://link.springer.com/book/10.1007%2F978-94-011-0487-6 .
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://zeus.theochem.tuwien.ac.at/pipermail/wien/attachments/20160306/d72278ff/attachment.html>