Dear Mohamed,

By structural relaxation, do you mean geometry optimization? May I ask you why do you care about the band gap for unoptimized structures? What sense does it make?

No, dear vladimir, I am not talking about optimization, but rather relaxation of internal positions. Other than the cubic systems, we must always relax the positions of the internal atoms before the optimization of the minimal energy vs volume. After the optimization, we take the equilibrium lattices parameters and put them in new case.struct for running the SCF in order to obtain the band gap. But when we relax the structure and run the SCF, we found a value of band gap smaller than without relaxation. My question is the physical explanation of this decrease.

I see: you are talking about PBC. I think it works the same way as for molecular systems (cluster models). During geometry optimization, electrons gradually accommodate in a more and more optimal way, which means that LUMO energy decreases (and I suppose, HOMO energy increases simultaneously) => band gap decreases.

Dear Mohamed ,

Your question is not correct, because the considered structure before structural relaxation is not the ground state structure. So, you may obtain the band gap larger or smaller than ground state value.

Dear Jaafar the Ab-initio calculations are all for the ground state, the excited states are not considered for these types of calculations. My question is correct, and with my experience, the band gap decreases always after relaxation. You can see the response posted by Vladimir

Dear Mohamed ,

My mean was not about excited state,.

You considered an initial lattice constant or atomic positions, after force minimization or energy optimization, both of them change. So, this new structure is your ground state structure. We can not compare the band gap of relaxed and unrelaxed structure. It does not have physical meaning.

But, you can consider strain effect (Compressive and tensile strains) on band gap variation of materials. This topic is correct "Why the energy band gap change when we apply strain ?"

Dear Jaafar

Can you give me more details about the strain effects on the band gap on monolayer.

(Uniaxial and biaxial strain).

Thank you very much for all

Dear Elmustapha,

Please study my recent publications.

https://scholar.google.com/citations?user=nSB1CKEAAAAJ&hl=en

Dear Jaafar

Thank you very much.

Feddi

Dear Mohamed;

I totally agree with the opinion presented by Jaafar Jalilian.

Best Regards