## The triplet excitations

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### The triplet excitations

Dear users and developers:

When calculating triplet excitations, we can do this by two methods respectively.
i) set BSENGexx = 0
ii)calculating a ground state including spin orbs coupling

I would like to know if I need to set BSENGexx equal to zero when using method two.

I will be very grateful that if anyone can give me a final confirmation.

Thank you !

Shen Chenhai
Chenhai Shen
Department of Physics, Henan Normal University
Xinxiang, 453000
shenchenhai

Posts: 10
Joined: Thu Mar 29, 2018 11:41 pm

### Re: The triplet excitations

Dear Shen,
Note, method 1 is a triplet calculation, while method 2 it is not. When considering spin-orbit coupling you are in the non collinear regime so you cannot define singlets or triplets.

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: 1950
Joined: Tue Mar 17, 2009 2:23 pm

### Re: The triplet excitations

Dear Daniele,
Thank you very much for your help, but I still have some questions about computing triplet excitations.
In the post (viewtopic.php?f=28&t=1385&p=6604&hilit=triplet#p6604), your answers to the singlet-triplet splitting calculations are as follows

Please note that singlet triplet splitting can be calculated by calculating separately singlet and triplet excitations.
Triplet excitations are calculated by omitting the exchange term in the excitonic Hamiltonian. See e.g. Rohlfing and Louie PRB 62, 4927 (2000).
It is also possible to consider non collinear spin, by calculating a ground state including spin orbs coupling and the BSE will be solved in a spinorial space.

I don't quite understand the meaning of the last sentence. Does it mean that if the ground state calculation contains SOC, the yambo calculation can directly produce the triplet excitation eigen-energies?

I will be very grateful that if you can give me a final confirmation.

Best,
Chenhai Shen
Chenhai Shen
Department of Physics, Henan Normal University
Xinxiang, 453000
shenchenhai

Posts: 10
Joined: Thu Mar 29, 2018 11:41 pm

### Re: The triplet excitations

Dear Chenhai,
I don't quite understand the meaning of the last sentence. Does it mean that if the ground state calculation contains SOC, the yambo calculation can directly produce the triplet excitation eigen-energies?

No the two sentences in my old post are not related.
It is also possible to consider noncollinear spin, by calculating a ground state including spin orbs coupling and the BSE will be solved in a spinorial space.

This is a feature of yambo which can deal with spin-orbit coupling by solving the BSE in spinorial space, but these are not triplet excitation as in a non-collinear spin description you do not have triplet and singlet anymore.

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: 1950
Joined: Tue Mar 17, 2009 2:23 pm

### Re: The triplet excitations

Dear Daniele,
It solved completely my questions. Now, everything is clear.
Sincerely,
Chenhai Shen
Department of Physics, Henan Normal University
Xinxiang, 453000
shenchenhai

Posts: 10
Joined: Thu Mar 29, 2018 11:41 pm