BSE with finite momentum Q

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Franz Fischer
Posts: 43
Joined: Wed Jul 20, 2022 9:36 am

BSE with finite momentum Q

Post by Franz Fischer » Wed Nov 01, 2023 3:04 pm

Hi all,

I did some finite center of mass momentum (Q) BSE calculations.
To my surprise the number of exchange G-vectors used in order to compute the G-sum for the electron-hole exchange matrix elements increased drastically from 23005 (for Q=0) to 111631 (for Q .ne. 0).
Furthermore I could not fully understand how the FFT grid for Q .ne. 0 (38 x 38 x 287) can get bigger than the FFT grid given by Quantum Espresso (36 x 36 x 270).

Why is this the case?

In the file "src/bse/K_driver.F" I found these 4 lines:

Code: Select all

if (.not.iq==1) then
	iGo_BSE=maxval((/4*qindx_B_max,qindx_X(iq,:,2)/))
       	iG_BSE =max(BS_n_g_exch,BS_n_g_W)
endif
Why do you multiply the qindx_B_max by 4? Can you elaborate on this?


Thanks in advance.
Best,
Franz
Franz Fischer
PhD student / IMPRS-UFAST fellow
Institute of Physical Chemistry
University of Hamburg

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Davide Sangalli
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Re: BSE with finite momentum Q

Post by Davide Sangalli » Thu Nov 02, 2023 6:29 pm

Dear Franz,
I checked the code, and I think you can safely remove the lines you reported in your message and recompile the code.

Also, if you want, you can test this branch of the code which has some improvement for BSE.
https://github.com/sangallidavide/yambo ... -dichroism

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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