Dear all,
I want to obtain the twodimensional polarizability alpha_2D from RPA or BSE calculations. I noticed that there are two ways to do that: 1) calculate it directly from RPA, as in "PHYSICAL REVIEW B 84, 085406 (2011)"; 2) do BSE calculations and get the binding energy, then build a reliable lowenergy model and adjust alpha_2D to match the model and firstprinciple binding energies, as in Nature Nanotechnology volume 15, 367–372 (2020).
I tried these two methods in 2D hBN systems. What confused me is that for a 18*18*1 calculation, the ALPHARe(\omega=0) (ALPHA0Re also) in orpa.alpha_q1_inv_rpa_dyson file is different from the one in obse.alpha_q1_diago_bse file. Here are my questions:
1) I think the reason for this discrepancy is that I set BSE bands to 26, but set BndsRnXd in rpa to 160. Is that true?
2) If it's true, does it mean that the correct alpha_2D comes from the RPA output file?
The input and output files are attached.
Best wishes,
Huaiyuan
How to obtain the correct 2D polarizability alpha_2D?
Moderators: Davide Sangalli, andrea.ferretti, myrta gruning, andrea marini, Daniele Varsano, Conor Hogan

 Posts: 20
 Joined: Wed Sep 02, 2020 11:10 am
How to obtain the correct 2D polarizability alpha_2D?
You do not have the required permissions to view the files attached to this post.
Huaiyuan Yang
Peking University
Beijing, China
Peking University
Beijing, China
 Daniele Varsano
 Posts: 3525
 Joined: Tue Mar 17, 2009 2:23 pm
 Contact:
Re: How to obtain the correct 2D polarizability alpha_2D?
Dear Huaiyuan,
the two calculations differs because you are using two different levels of theory (RPA and BSE).
Parameters are different, but this is not a problem if they are at convergence (this is something you need to check)
Finally, please note that a BSE calculation should be done on top of QP energies and not KS energies so you need to perform either a GW calculation and run BSE on top including the QP energies via the KfnQPdb keyword, or model them by using a scissor operator using the KfnQP_E keyword.
Best,
Daniele
the two calculations differs because you are using two different levels of theory (RPA and BSE).
Parameters are different, but this is not a problem if they are at convergence (this is something you need to check)
Finally, please note that a BSE calculation should be done on top of QP energies and not KS energies so you need to perform either a GW calculation and run BSE on top including the QP energies via the KfnQPdb keyword, or model them by using a scissor operator using the KfnQP_E keyword.
Best,
Daniele
Dr. Daniele Varsano
S3CNR Institute of Nanoscience and MaX Center, Italy
MaX  Materials design at the Exascale
http://www.nano.cnr.it
http://www.maxcentre.eu/
S3CNR Institute of Nanoscience and MaX Center, Italy
MaX  Materials design at the Exascale
http://www.nano.cnr.it
http://www.maxcentre.eu/

 Posts: 20
 Joined: Wed Sep 02, 2020 11:10 am
Re: How to obtain the correct 2D polarizability alpha_2D?
Dear Daniele,
When all the parameters are converged, results from RPA and BSE are the same. Is that right? I noticed that in obse.alpha_q1_diago_bse file, DipBands are the same as BSEBands, to be "2  6". Is that a problem to the result?
Best,
Huaiyuan
When all the parameters are converged, results from RPA and BSE are the same. Is that right? I noticed that in obse.alpha_q1_diago_bse file, DipBands are the same as BSEBands, to be "2  6". Is that a problem to the result?
Best,
Huaiyuan
Huaiyuan Yang
Peking University
Beijing, China
Peking University
Beijing, China
 Daniele Varsano
 Posts: 3525
 Joined: Tue Mar 17, 2009 2:23 pm
 Contact:
Re: How to obtain the correct 2D polarizability alpha_2D?
Dear Huaiyuan,
If you want RPA is the same to the BSE when the xpart only of the kernel is considered, in this case it is just a change of basis set.
Note that in order to obtain the same results, the parameters (bands, and G vectors (in exchange for BSE, Gblk in RPA)) should be the same, moreover you need to pay attention to the timeordering.
Best,
Daniele
No, they are not, as they contain different physical insight, and RPA misses the electronhole attraction interaction (excitons).When all the parameters are converged, results from RPA and BSE are the same. Is that right?
If you want RPA is the same to the BSE when the xpart only of the kernel is considered, in this case it is just a change of basis set.
Note that in order to obtain the same results, the parameters (bands, and G vectors (in exchange for BSE, Gblk in RPA)) should be the same, moreover you need to pay attention to the timeordering.
Best,
Daniele
Dr. Daniele Varsano
S3CNR Institute of Nanoscience and MaX Center, Italy
MaX  Materials design at the Exascale
http://www.nano.cnr.it
http://www.maxcentre.eu/
S3CNR Institute of Nanoscience and MaX Center, Italy
MaX  Materials design at the Exascale
http://www.nano.cnr.it
http://www.maxcentre.eu/