Underestimation of bandgap

[abhishek77492652]

Why DFT-GGA and DFT-LDA underestimate the bandgap  of  non-metals ?

[Daniele Stradi]

Because of the so-called self-interaction error due to the use of approximate exchange-correlation functionals. See e.g.:

J. P. Perdew and Alex Zunger
Self-interaction correction to density-functional approximations for many-electron systems
Phys. Rev. B 23, 5048
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.23.5048