I have some questions about how to get the meaningful dielectric functions when using YAMBO to do the GW-BSE calculation for some 2D materials. I have already known how to use RIM and CUT settings to treat low-dimensional systems, and have successfully got the 2D polarizability alpha. Below are my questions:

1. Is the output file o-GW.eps_q1_diago_bse of GW-BSE calculation containing the 2D dielectric functions \epsilon? Does this file is produced by using the formula \epsilon(q) = 1 + 2 \pi * \alpha_2D * |q|. If not this case, what is this file?

2. How to get the optical conductivity or absorbance using 2D polarizability alpha? I note that the tutorial https://www.yambo-code.eu/wiki/index.ph ... al_systems written,

Does this mean we can directly use the imagine part of alpha to describe optical conductivity or absorbance? Is there any reference formula for me to get it?For this reason yambo produces another file which contains the macroscopic polarizability (called here 'o-2D_WR_WC.alpha_q1_diago_bse') which is a well defined quantity strictly related to optical conductivity or absorbance.

3. If I use yambopy to get the BSE optical spectra using yambopy/tutorial/databases_yambopy/exc_abs_plot.py, what the obtained \chi is? In the tail of the definition of python function get_chi, the author comment that

So I get the epsilon instead of polarizability?chi = 1. + chi*cofactor #We are actually computing the epsilon, not the chi.

Please give me some advice. Thanks in advance.