The anisotropy in BSE calculation

Deals with issues related to computation of optical spectra, solving the Bethe-Salpeter equation.

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

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Guo_BIT
Posts: 28
Joined: Tue Jun 06, 2023 2:55 am

The anisotropy in BSE calculation

Post by Guo_BIT » Tue Jan 23, 2024 3:36 am

Dear developers:
Currently, I would like to perform direction-specific BSE calculations in a two-dimensional system, just as done in Fig. 3d of the article
https://www.nature.com/articles/s41563-022-01285-3
However, I have a few questions below:
(1)In my GW calculations, I set

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% LongDrXp
 1.000000 | 1.000000 | 0.000000 |
to calculate static screening in plane since my system is 2D. Then, in my BSE calculations,

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% BLongDir
 0.000000 | 1.000000 | 0.000000 |
was set to get the absorption spectra along the y direction. Is my process correct?

(2)When performing BSE calculations without considering anisotropy, I will set

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% BLongDir
 1.000000 | 1.000000 | 0.000000 |
which means calculating along the 45° direction in the xy plane. So, is it correct to set

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% BLongDir
 0.866 | 0.5 | 0.000000 |
if I want to perform BSE calculations when deviating 30 degrees along the x direction?

Best wishes,
Guo
Jingda Guo
Beijing Institute of Technology

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Daniele Varsano
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Re: The anisotropy in BSE calculation

Post by Daniele Varsano » Tue Jan 23, 2024 9:49 am

Dear Jingda,

Yes, both 1) and 2) are correct!

Best wishes,

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/

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Davide Sangalli
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Re: The anisotropy in BSE calculation

Post by Davide Sangalli » Thu Jan 25, 2024 1:49 pm

Let me add that, in case your system is not isotropic in plane, you can use the following input for GW

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% LongDrXp
 1.000000 | 1.000000 | 0.000000 |
%
OptDipAverage
The logical `OptDipAverage` triggers an averaging procedure for the q=0 term of the screening.
The average is done over the Cartesian directions selected in the variable `LongDrXp` , e.g. in this case x=(1,0,0) and y=(0,1,0).
Without the logical, the screening at q=0 is instead computed along the direction (1,1,0)

This feature is available from version 5.1

Best,
D.
Davide Sangalli, PhD
CNR-ISM, Division of Ultrafast Processes in Materials (FLASHit) and MaX Centre
https://sites.google.com/view/davidesangalli
http://www.max-centre.eu/

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