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I've done a GW+BSE calculation for obtaining the dielectric function of a 2D material, following the tutorial (https://www.vasp.at/wiki/index.php/Diel ... _using_BSE). The imaginary and real parts of the dielectric function are represented as 6 different contributions (xx,yy,xx,xy,xz,yx). My question is, if I want to get the "overall" imaginary part of the dielectric function, how do I combine these components. Is it just a summation?, an average? Joining the components by the sqrt of the sum of squares?

Hi Diego Ontiveros,

Usually, in 3D systems, the spectrum is presented as an average of 3 components of the dielectric tensor (xx+yy+zz)/3. In 2D systems, the zz component is very small, so the dielectric tensor out-of-plane (zz) and in-plane (xx+yy)/2 are presented separately, but in principle it depends on the polarization of the incident light.
You can find more detail in I. Guilhon, et al., Phys. Rev. B 99, 161201 (2019)