Dear Shabnam Ahadzadeh,

First of all, Ultrasoft Pseudopotentials is more accurate than norm-conserving in the lattice parameter up to 0.25%. So you got the correct and more accurate bandgap of ZnO nanotubes by using Ultrasoft.

For the Optical properties, in my opinion, I think norm-conserving is the one can calculate it because it's constructed to enforce all wavefunctions inside and outside the cut-off radius, while for ultrasoft it's constructed to enforce wavefunctions inside the cut-off radius only, and are zero outside.

You can obtain the correct bandgap by using Ultrasoft, and for optical properties, you need to use norm-conserving in different calculation and it will be correct.

Hey dear Qusai Ibrahim,

Thanks for your response

I have one more question regarding norm-conserving pp. I realized the code that I downloaded from the Q.Espresso P.P table doesn't contain the value of minimum cutoff for wave functions and the minimum cutoff for charge density, so I think the optical properties of ZnO nanotubes relate to them...

Therefore can I ask you to help me with choosing sufficient and proper value for them?

best regards

Dear Shabnam Ahadzadeh,

The minimum value of Ecut(wfc) (wave function) should be equal to 600.00 eV which equal to approximately 44(Ry), while for Ecut(rho) (charge of density) should be exactly 4 times higher than the Ecut(wfc) which equal to 176(Ry). However, sometimes, proper cutoff energy is written when creating the corresponding Pseudopotential file.

I hope this helps.

Best regards.

Professor Behnam Farid

Thank you for your clear and remarkable response.

What do you recommend for obtaining optical properties? As we noted, one of the P.Ps gets a good bandgap while it doesn't give us optical properties and another one gives optical properties but the provided bandgap is not good.

In your point of view, what can I do for obtaining optical and electronic properties of nanotubes with Q.espresso DFT calculation?

best regards

Prof. Behnam Farid

I'm very grateful for your excellent explanations. It will be so useful to me.

Also, I appreciate the references you provided for me.