You can define the optical gap in many ways (e.g. Tauc, IPR, first allowd- transition, etc.). But that's not my concern.
I reformulate the question: how can I have interband absorption at E<Egap? R In date 4/3/11 15:26:30, Gregorio García Moreno wrote: > Optical bandgap is obtained from UV/Vis spectra as the lowest transition > (or excitation) energy from the ground state to the first dipole-allowed > excited state. It is an implicit assumption that the lowest singlet > excited state can be described by only one singly excited configuration, > wherein an electron is promoted from the HOMO to the LUMO > However, the energy difference HOMO LUMO could consider more of one > electron. > Even though, as Abhishek says, DFT can provide trend > > El 3/4/2011 3:10 PM, Abhishek Sharma escribió: > > If you used DFT method to obtain band structure than always remember a > > concept that DFT underestimate band gap, however it can provide trend > > in variation of band gap for different systems. > > > > Abhishek Sharma > > > > On Fri, Mar 4, 2011 at 7:31 PM, Roberto Guerra <[email protected] > > > > <mailto:[email protected]>> wrote: > > Hello, > > > > I calculated the band structure of bulk amorphous silicon (64 atoms > > tetrahedrically coordinated). I found an HOMO-LUMO band gap of > > about 0.7 eV. > > Then I performed the OpticalCalculation, but I have that the > > resulting imaginary dielectric function has a peak at E<0.7. How is > > this possible? I > > expect that each valence-to-conduction transition must be larger > > than the > > bang-gap! > > > > Roberto
