Analysing results

[Ravneet Kaur]

How can we find out the energy gap from i) Transmission Spectrum ii) Bandstructure
and what is the difference between Bandstructure and Molecular Energy Spectrum?

[zh]

Band structure is referred to a bulk system with periodicity. For the more details, you had better read a textbook on the solid state physics to understand the basic background.
Molecular energy spectrum---> the digram of energy levels of a molecule or an isolated system (e.g., cluster).

The band gap can be read from the band structure by looking at the bottom of conduction band and the top of the valence band. If you want to read the band gap from the transmission spectrum,  the transmission coefficient at E_F should be zero, and then you may look at the first energy above E_F with non-zero transmission coefficient and the first energy below E_F with non-zero transmission coefficient.

[Ravneet Kaur]

Thanks for the valuable information Sir but m still not clear why treating GNRs as bulk configuration computes bandstructure and there is no option for banstructure if the same bulk GNR is taken in device mode. Does Device mode make the system to be treated a s a molecule? No text book has these basic answers 

Moreover here I have a TS : can u please explain how can I calculate its Eg?

[zh]

band structure cannot be referred to a device mode.  Also the device mode cannot be treated as a molecule.

The results in your attached figure indicates no band gap exist for the system under your study.