I am currently working with a perovskite material which shows forbidden transitions. Older studies used bigger values of $\alpha$ in HSE06 calcualtions(to match with experimental Band Gap) but a recent study showed that this is not the case. Rather the electronic BG is around 0.98 and optical one is around 1.3. This got me thinking how can we differentiate between the two using DFT? I believe the answer should lie in absorption coefficient or the dielectric constant.
In general how can one determine the Optical band gap?
A few possible solutions that I have been thinking about
- Tauc plot - we could use the absorption coefficient calculated in order to calculate the BG, obviosuly this is motivated from general experimental methodology.
- Using absorption coefficient - One can look at the critical values of absorption coefficient in order to see where the Optical BG might be. An example is this study.
- Using imaginary part of dielectric constant - The study in point (2) also says that BG can be determined by looking at the critical BG of imaginary part of dielectric constant but they obtain a different value as compared to Optical BG using (2).
- Using Penn's model($\epsilon(0) \approx1+(\hbar\omega_p/E_g)^2$) one can relate the static dielectric function and the BG. Again (2) as a example.
Furthermore why don't people generally talk about this? Is it because calculation of Optical properties is more computationally costlier than calculation of Electronic ones?