## negative self-energy

Concerns issues with computing quasiparticle corrections to the DFT eigenvalues - i.e., the self-energy within the GW approximation (-g n), or considering the Hartree-Fock exchange only (-x)

Moderators: Daniele Varsano, andrea.ferretti, andrea marini, myrta gruning

### negative self-energy

Hi all,

In full-frequencies GW calculation, I find imaginary part of self-energy is negative at some states. I think it is a numerical problem instead of a physical one.
As try, I play with GDamping and dScStep parameters, and find it is hard to eliminate the negative self energy exactly.
So what is problem here? How to ensure that all self-energy is positive?

Best,
Weiqing
Weiqing Zhou
Phd student
Wuhan University
Wuhan, Hubei, 430072
Weiqing Zhou

Posts: 48
Joined: Thu Sep 06, 2018 7:57 am
Location: Wuhan University

### Re: negative self-energy

Dear Weiqing,
I do not know if this reflects your case, but for conventions lifetime of electrons(hole) is set positive(negative).
See here for an example:
http://www.yambo-code.org/tutorials/Rea ... /index.php

Best,
Daniele
Dr. Daniele Varsano
S3-CNR Institute of Nanoscience and MaX Center, Italy
MaX - Materials design at the Exascale
http://www.nano.cnr.it
http://www.max-centre.eu/

Daniele Varsano

Posts: 2026
Joined: Tue Mar 17, 2009 2:23 pm

### Re: negative self-energy

Dear Daniele,

Many thanks!

Acutally, I have noticed the Lifetime Tutorials in Yambo main page before. What I use is Version 4.3.0 Revision 129, and I think there are some updates about self-energy compared to the version used in Tutorials. Different from the result shown in Tutorials, the self-energy from Yambo 4.3.0 is all positive for electron and hole. I post my Al result in attachment.
And From Yambo 4.3.0, I still got some negative self-energy, which is my problem.

Best,
Weiqing
You do not have the required permissions to view the files attached to this post.
Weiqing Zhou
Phd student
Wuhan University
Wuhan, Hubei, 430072
Weiqing Zhou

Posts: 48
Joined: Thu Sep 06, 2018 7:57 am
Location: Wuhan University

### Re: negative self-energy

Dear Weiqing,
I see, can you post the report file of your calculation where you find the problem, I can't see it from the file you post or am I missing something?

Best,
Daniele
Dr. Daniele Varsano
S3-CNR Institute of Nanoscience and MaX Center, Italy
MaX - Materials design at the Exascale
http://www.nano.cnr.it
http://www.max-centre.eu/

Daniele Varsano

Posts: 2026
Joined: Tue Mar 17, 2009 2:23 pm

### Re: negative self-energy

Dear Daniele,

Sorry for my negligence. Last post, I just upload Al result to compare with Tutorials.

The negative self-energy appears in a semi-metal I study recently. Actually, self-energy is all positive and look nice when GDamping is default value, namely 0.1 eV. I think GDamping, as a small value, should be as small as possible theoretically and self energy should get converged as GDamping decreases. Then I do GW calculation with different GDamping (0.1 eV, 0.01 eV and 0.001 eV ) and the self-energys with different GDamping as a function of KS energy are plotted in attachment. It is clear that self-energy approach convergence as GDamping decreases, while negative self-energy appear when GDamping is smaller than 0.01 eV. How to eliminate the negative self-energy with a small GDamping?

Best,
Weiqing
You do not have the required permissions to view the files attached to this post.
Weiqing Zhou
Phd student
Wuhan University
Wuhan, Hubei, 430072
Weiqing Zhou

Posts: 48
Joined: Thu Sep 06, 2018 7:57 am
Location: Wuhan University

### Re: negative self-energy

Dear Weiqing,

not sure this is at the origin of the issue, but typically lowering the broadening parameters should go together with an increase in the kpt grids.
I think that if you just lower the dumping, you reach a point when your calculation is not converged wrt kpts.
Personally, I would follow an algorithm where, given a broadening value one makes sure to converge kpts, then one can lower the broadening and re-check for kpts and so on

Andrea
Andrea Ferretti, PhD
CNR-NANO-S3 and MaX Centre
via Campi 213/A, 41125, Modena, Italy
Tel: +39 059 2055322; Skype: andrea_ferretti
URL: http://www.nano.cnr.it
andrea.ferretti

Posts: 97
Joined: Fri Jan 31, 2014 11:13 am