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Hi
I'm trying to calculate excitation energy for silicene. (Definition of excitation energy is the first peac in spectre of dielectric function), in the paper as follow [PhysRevB.88.045203] they worked by third peac.
2) For binding enery which is defined as the difference between the excitation energy and the QP energy. in silicene (zero-gap semimetallic), binding energy is equal to the excitation energy.
Thank you

Dear Fatima,
I'm sorry but I do not have understand exactly what is your question, could you try to explain it better?
Anyway I do not recognize your definition of binding energy, the binding energy is defined as the difference between the QP gap (free electron and hole) and the excitation energy (bound electron-hole). In case of 0 gap system, I can't find a way to define a binding energy with this definition, even if you can have confined excitation so, somehow bounded. May be you can have a look to the difference between an independent quasi particle spectra and the BSE spectra.

Best,
Daniele