Yambo Community Forum

Dear developers,

I'm calculating the band structure of 1H-MoS2 monolayer using G0W0 corrections (Yambo-5.0.4) and parameters from PhysRevB.88.045412, but it results in a large gap of about 3 eV, compared to 2,41 eV from the paper.
To me, I think I used good parameters and PP (SG15 ONCV), the only parameter I'm concerned about is the cut box size, from discussions in the forum I use 39 a.u. (my system is 21 Angstrom in Z direction).
I realize the gap would be significantly reduced by reducing the cut box to about 8 a.u. I don't know what should I do.
I wonder if you'd mind taking a look at my QE inputs and Yambo reports?

Best,
Minh

Dear Minh,

the cutoff size is correct. Looking at your input:
VXCRLvcs is a very small value, you should use all the g vectors of the density (77001 RL).
50 bands also seem to me it is not at convergence as well as NGsBlkXp
Aso the k point sampling should be carefully checked.

Anyway, someone expert on the system can probably provide you with more details.

Best,
Daniele

Dear Minh,

I agree with Daniele.

In order to get reasonably converged results, I think that the kpoint grid should be between 39x39x1 and 42x42x1.
In addition, for NGsBlkXp you should put at least 100 bands, probably more.

The band gap dependence on the cutoff that you observed usually happens when the kgrid is not well converged. Indeed, the various convergence parameters are not exactly independent from one another but have a degree of interdependence.

Cheers,
Fulvio

Dear Minh,

please have also a look to FIg.4 of Phys. Rev. B 94, 155406.
We have recently implemented a very efficinet scheme to accelerate convergence wrt k points for 2D materials that will released very soon.

Best,
Daniele

Dear Fulvio and Daniele,

Thank you for helping, after increasing K-grid to 36x36 and changing lattice parameters to relaxed values I get a gap of about 2.65 eV.

I want to include excitonic effects in nonlinear optical calculations for the material, but as I've read monolayer MoS2 has a large exciton binding energy.
So I don't know if it is sufficient when I only include the exchange term <cvk|V|c'v'k'> and ignore the screened Coulomb <cvk|W|c'v'k'>?

In Yambo to Wannier90 procedure, I see there is an update in the overlap matrices from DFT to GW.
I don't understand how it's done because GW uses KS orbitals as the basis.
With this misunderstanding, I'm wondering should there be an update for Coulomb matrix elements?
Because I think <cvk|V|c'v'k'> is calculated with KS states and if the overlap matrices are changed using some QP states, Coulomb matrices should also use these QP states.
I'm looking for your help.

Best,
Minh

Dear Minh,

So I don't know if it is sufficient when I only include the exchange term <cvk|V|c'v'k'> and ignore the screened Coulomb <cvk|W|c'v'k'>?

Of course in this way you are missing the excitonic effects.

Because I think <cvk|V|c'v'k'> is calculated with KS states and if the overlap matrices are changed using some QP states, Coulomb matrices should also use these QP states.

In GoWo procedure wavefunctions are not changed, there is the assumption that Psi_QP ~ Psi_KS and in general the approximation is valid.

Best,
Daniele

Dear Daniele,

I still don't understand why overlap matrices are updated if we assume that Psi_QP ~ Psi_KS.
I think if we have the same k-grid and band numbering, overlap matrices will stay the same as DFT overlap matrices. Is that correct?

Best,
Minh

I still don't understand why overlap matrices are updated if we assume that Psi_QP ~ Psi_KS.
I think if we have the same k-grid and band numbering, overlap matrices will stay the same as DFT overlap matrices. Is that correct?

G0W0 can change the ordering of the eigenvalues, that means that the eigenvalues (and so the eigenstates) might not be ordered in energy anymore.
This is a problem for Wannier90, that assumes energy ordering in many steps, for instance when the energy windows are used.
Hence we reorder the indices of the KS orbitals: the orbitals themselves are unchanged, it's just the ordering that is updated.

HTH