What are the variables Blongdir and LongDrXS?

Deals with issues related to computation of optical spectra in reciprocal space: RPA, TDDFT, local field effects.

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What are the variables Blongdir and LongDrXS?

Post by milesj » Tue Feb 14, 2023 12:59 am


I've been going through the tutorials on the wiki page, and now am trying to run a BSE computation to get the optical spectrum for a material I'm interested in. In the kernel and diago solver steps, we encounter two variables Blongdir and LongDrXS, which both indicate the cartesian direction of something. What do these indicate? The variables page on the wiki does not explain this well, and I'm not sure where else to look. It seems like both of these are related to the direction of some electric field, but beyond that I have no clue what they could mean.
Any simple explanation or directions toward something that explains what these are would be very helpful.

Miles Johnson
California Institute of Technology
PhD candidate in Applied Physics

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Daniele Varsano
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Re: What are the variables Blongdir and LongDrXS?

Post by Daniele Varsano » Tue Feb 14, 2023 9:01 am

Dear Miles,

yes, they indicate the polarization of the perturbing field in linear response.

LongDrXS is used in the calculation for the static screening to build the kernel (lim q->0 in Eq. 8 of the 2009 Yambo paper.
Blongdir is the q->0 limit for the absorption spectrum in BSE (Eq. 23 in the same paper).

Dr. Daniele Varsano
S3-CNR Institute of Nanoscience and MaX Center, Italy
MaX - Materials design at the Exascale

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