Dear Abhishek, I'm not troubled at all for the underestimation of the gap, but for the fact that any absorption-related transition must go from the valence to the conduction band, and therefore it must have an anergy larger than the gap. Instead I find a peak at E<Egap! There must be something wrong.
R In date 4/3/11 15:10:12, Abhishek Sharma wrote: > 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]> 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
