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)

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 .
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