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Dear Daniele,
I have been bothered by this problem.I have calculated the out-of-plane absorption spectrum of a two-dimensional ferroelectric material, and set the light direction to (0 0 1) and (0 0 -1) respectively. Logically speaking, for these two directions, the absorption intensity is different. However, the absorption spectrums I calculated are the same.
I followed this step to calculate and added the -r to generate a truncated coulomb potential. Step 1: Static screening Step 2: Bethe-Salpeter kernel Step 3: Include previous quasiparticle (GW) results.
I don't know where the problem is，Can you take a look at it for me？
The report file is attached below.

Best wishes,
Liu

Dear Liu,

have calculated the out-of-plane absorption spectrum of a two-dimensional ferroelectric material, and set the light direction to (0 0 1) and (0 0 -1) respectively. Logically speaking, for these two directions, the absorption intensity is different. However, the absorption spectrums I calculated are the same.

As far as I understand eps_{xx}=eps_{-x-x} by definition.
Indeed one computes the dipoles <i|x|j> and then ~eps_{xx}

How can the two be different?

Best,
D.