I am currently engaged in a research project identifying whether some two-dimensional materials are excitonic insulators (EI). The two-dimensional polarizability alpha_2D is an important parameter to describe the screened Coulomb interaction in some theoretical models of EI, which also can be find in the excellent paper

**Nature Nanotechnology volume 15, 367–372 (2020)**. So I wonder how to get a meaningful and correct 2D polarizability alpha_2D using Yambo, because I knew that Yambo can calculate that parameter. Here are my questions:

(1) I noticed that the output file

**o.alpha_q1_diago_bse**obtained using the truncated coulomb potential in a BSE calculation contained two types of alpha_2D (alpha and alpha_o) which were very different in value. Is the value of alpha the result of the BSE calculation, and alpha_o the result of the GW calculation? Or are they should be the same?

(2) In theoretical calculations, just like paper

**Nature Nanotechnology volume 15, 367–372 (2020)**, which value should we use, the BSE result, the GW result, or the result which we should calculate in RPA? I tried both BSE and RPA calculation in yambo, but the RPA result

**o.alpha_q1_inv_rpa_dyson**also contains two types of alpha_2D (alpha and alpha_o) and those results are different from the BSE result. What's worse is that the Re(alpha) in the RPA result are negative, is this reasonable? There are so many alpha_2D, so I'm not sure which one should i choose...

(3) As we can seen, the two-dimensional polarizability alpha_2D is energy-dependent, should we choose the alpha_2D at zero energy to use?

The input and output files are attached.

Best wishes,

Haozhe