[Pw_forum] Convergence or band-gap problem in orthorhombic CsPbBr3

Federico Iori federico.iori at u-psud.fr
Thu Sep 8 10:19:45 CEST 2016

If your calculation GW+BSE are well converged, probably your results are not wrong at all: if your BSE optical gap is in agreement with the experimental one, it can be related with some cancellation between the opening of the electronic gap by the GW correction and its following reduction due to excitonic effects when calculating the excitonic contribution. 

What about the optical experimental gap thou? 

I do not know if y our input file is the one to prepare the yambo run or not. In this case, I think the run should be with QE a NSCF calculation with large number of empty band and different mesh of increasing k point in order to check the convergence for the screening calculation (GW part) and the BSE part afterwards. 
Did you converged correclty the W screening even wr.t. the G vector ? 

Remeber the BSE converges with few bands but usually it requires more k point than the GW part. 


Federico IORI 
Marie Curie Fellow 
Laboratoire de Physique des Solides 
Bâtiment 510 - Rue André Rivière 
91400 Orsay 

----- Mensaje original -----

De: "Christoph Wolf(신소재공학과)" <chwolf at postech.ac.kr> 
Para: "pw forum" <pw_forum at pwscf.org> 
Enviados: Jueves, 8 de Septiembre 2016 4:11:49 
Asunto: Re: [Pw_forum] Convergence or band-gap problem in orthorhombic CsPbBr3 

Dear Frederico! 

The experimental band gap is 2.3-2.4 depending on the method used; my band gap is less than 0.5 eV – rather drastic underestimation. For the orthorhombic phase there are two or three references, but they do not agree well in terms of the location of the gap (sometimes gamma, sometimes R-point) and I am not sure if they applied scissors (it is not stated in the manuscript). 

The interesting thing is that the BSE absorption spectrum for the cubic cspbbr3 (a=5.6 A) agrees quite well with my experimental values but the XRD pattern suggests I have o-CsPbBr3. I am following the YAMBO GaSb tutorial (it calculates exactly what I want to calculate); but I am curios where I went wrong with my scf/nscf calculation… 

Thank you for your suggestions! 



Christoph Wolf 

POSTECH university 

Dep. Materials Science and Engineering 

Chung-am ro 77 

Pohang, Gyungbuk 

Republic of Korea 


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