You may have to change verbosity to 'high" to print out all the eigenvalues and find it manually, but I believe in that case it should print the band gap or at least the fermi level. 

However if quantum espresso finds your system to be insulator it will not calculate any thing above the fermi level and you will have to add those levels in manually.   Although if you are seeing a band gap in your nscf calculations I assume its not an an actual insulator, but something that has a bit of smearing at the top or bottom of a band.

http://www.quantum-espresso.org/wp-content/uploads/Doc/INPUT_PW.html#idp27136

http://www.quantum-espresso.org/wp-content/uploads/Doc/INPUT_PW.html#idp108000

plotting dos may work.

I would second what Joshua said in turning up the verbosity to "high" in quantum espresso. In that case it will print out all the eigenvalues and you can get the band gap.

You will also have to ask for more bands than just the occupied bands by setting the "nbnd" parameter in the &SYSTEM card:

http://www.quantum-espresso.org/wp-content/uploads/Doc/INPUT_PW.html#idp108000

A Yuanhai said, doing an SCF calculation to get the band structure is a bit expensive since you have to calculate all the kpoints. However, you have to do this step already and a few extra conduction bands in the SCF step is not so expensive. In fact, I almost always include a few conduction states because sometimes this helps with numerical stability (I have no idea why). Plus, then I know right from the SCF calculation if my results seem reasonable before I proceed. 

As people already recommended you should have high verbosity to the output file. 

If you include a smearing in the wave function occupation, Quantum Espresso will calculate a few extra bands.

In your calculation, you are assuming you know where the gap is located in terms of the k-point (at gamma?). If so, be sure that in the MP grid you use, the gamma point (or the point at which you want the gap) is included. 

Finally, considering that you have to run a SCF calculation in any case, this is not more expensive than running a nscf calculation. However, you will get information only on the point of the MP grid you use in the k-space. 

Thank you all for your comments and suggestions. They really help.

I think it depends on your system behavior. If you are working with a semi-conductor or an insulator, set verbosity=high and also the nbnd tag with respect to the number of valance electrons. QE will print your frontier orbitals in output. For the case of metal, it is more complicated and is beyond my experience.

I not getting right band gap Germanene using quantum expresso

Shahnewaz Mondal it is not clear what do you mean. Is your gap not consistent with experiments or with other calculations? Is your structure equivalent to the one reported in the literature? Is your calculation well converged with parameters comparable with those of other calculations?

Regards,

Roberto

My problem has been solved.Thanks for responding .

Can anybody suggest how to run band structure calculation by choosing just few top valence bands and few lower conduction bands in Quantum Espresso?

For what I know it is not possible to select the bands from the input of bands.x or plotbands.x

You can obviously decide to plot only a range of energies in any plotting software.

If your question is on reducing the weight of the calculation, pw offers a nbnd variable that is the total number of bands to be calculated. This must be at least the number of states to allocate all the electrons and a few conduction bands.

To reduce the calculation further you may need to select a pseudo potential that includes only a few electrons in the valence states. But in that case the risk of having non sensical results increases.

Thank you for the reply.

Can anyone please tell me how to get bond angle and bond length using BURAI software?

Can anyone please suggest what type of smearing and the value of degauss parameter should be used to reproduce the band structure of n-type metallic system.

Dear Sonali,

The answer is unfortunately no. And this is simply due to the fact that you should determine the value for the degauss and the type of smearing from

1) the system you want to investigate --

2) a convergence test on the quantity you want to calculate.

For the smearing type, I would probably try a standard gaussian and other options available in the code you want to use (for example, a Marzari-Vanderbilt is another common choice). Compare the two and see if you get similar results for the band structure. For the degauss value, choose a quantity you want to converge (the energy, the band gap?), start with a reasonably large of the degauss (the default value for your code) and then decrease it keeping the other parameters invariate. Compare the different results for value of the degauss until you are satisfied with convergence. As a rule, differences below 50 meV as to be treated carefully as many pseudopotentials will not offer an accuracy better than that.

I hope this helps.

Regards,

Roberto

PS: It might be useful if you tell the code you are using.

Thanks a lot Roberto D'Agosta for your response. I am using quantum espresso. I checked the band structure with "gaussian" as well as "Marzari-Vanderbilt smearing with degauss value 0.005 and the two band structures are similar. I am getting bands all over without any gap, whereas the system is n-type with a gap above the valence band. What value of degauss should I use to get a gap between VB and CB with fermi level in CB.

Dear Sonali Kakkar so if I understand correctly your problem, you get a band structure that is similar with the gaussian and MV smearing, but the physical system you want to investigate is a semiconductor with a finite gap, while your calculations returns a metal (with 0 gap).

This is admittedly a different and difficult problem: some potential checks

- You might try to reduce the smearing, but I admittedly would not expect much of an improvement.

- You might change the pseudopotentials, for solids usually PBESOL is recommended.

- You might check if relaxing or not relaxing the structure you get the opening of a band gap.

- You might check if including spin and spin-orbit coupling might open a gap. (but calculation gets heavier)

As another option, you might try to change the occupation to "tetrahedra_opt" but this forces the system in some fixed state and you might not be able to get correct results for other quantities. You will then need to confirm your results with more advanced methods, like GW.

I hope this helps,

Roberto

Thanks Roberto D'Agosta for your reply.