Optical spectrum problem in LiF tutorial

[hwtmorgan]

Dear VASP team,

I am trying to compute optical spectra with GW-BSE. I have followed the tutorial for LiF and I get the same dielectric function shown on the tutorial webpage. I have used that dielectric function to compute the absorption coefficient \alpha and transmission T according to the Beer-Lambert law:
T = exp (-\alpha * d) where d is the thickness of the sample

My transmission spectrum says that a 2mm sample of LiF is opaque to visible light, which is not true. I can get the correct transmission spectrum but only by using a very small d value. Can you suggest anything that might fix this problem?

Best wishes,
Harry

[marie-therese.huebsch]

Hello, I am sorry for the delay.
May I ask for clarification, when you say

My transmission spectrum says that a 2mm sample of LiF is opaque to visible light, which is not true.

do you mean that the computational result does not agree with experiment, or do you mean you cannot reproduce a numerical result?
Could you please provide a minimal reproducible? (c.f. posting guidlines)

Best regards,
Marie-Therese

[hwtmorgan]

Dear Marie-Therese,

Thank you for your reply. I have attached the files for the GW and BSE calculations.

I am trying to match the experimental transmission spectrum of LiF, see e.g. https://www.firebirdoptics.com/optical- ... if-windows.

I have also attached optical spectra from the BSE calculations:
my_BSE_TDA_imaginary_dielectric.png- this dielectric function matches the tutorial here https://www.vasp.at/tutorials/latest/bse/part2/#BSE-e07
my_BSE_TDA_absorption_spectrum.png - this is the absorption spectrum computed from that dielectric function

my_transmisson_spectrum_2mm.png - this is the transmission spectrum computed from the absorption spectrum according to the Beer-Lambert law with a thickness parameter d of 2 mm. It says that LiF is opaque to all wavelengths of light, which does not match experiment.
my_transmisson_spectrum_100nm.png - this is the transmission spectrum computed from the absorption spectrum according to the Beer-Lambert law with a thickness parameter d of 100 nm, which matches the experimental spectrum but the thickness is much too small.

Best regards,
Harry

[hwtmorgan]

Input files and spectra (images) in archive.