In the attachment, $S(\omega)$ is a transmission function, $\epsilon_1$ and $\epsilon_2$ are real and imaginary dielectric functions respectively. If we notice the equation carefully, we need $\epsilon (q, \omega)$ since the summation includes all the q points in the 1st BZ.

Normally if I do DFT using Quantum Espresso, I would get the dielectric function ($\epsilon$) over frequency ($\omega$) ranges. Are there anyway I would get dielectric function $\epsilon(\omega)$ at each q point?

The link of the paper related to the equation (9) (https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.14.024080)

Thank you in advance for your time.

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