After obtaining the Kohn-Sham orbitals from a plane-wave-based self-consistent-field calculation, the dipole matrix elements could be calculated in order to determine electro-optical properties such as the dieletric response of the material. For instance, Quantum ESPRESSO has two different approaches to accomplish it, namely the epsilon.x program, and TDDFPT (time-dependent density functional perturbation theory) with the turbo_eels.x program. The first one does not take into account electron-electron interaction, the latter does. Both of them are very time consuming and sometimes don't converge at all. It has been shown that such interaction is relevant for molecules (small ones, at least). What about periodic crystals, should I be concerned about it?
I think you should take care of all possible interactions to get close to the real picture. In periodic solids, there might be electron-hole interaction (solve BSE equation for it), el-phonon coupling, etc. Note that, QE epsilon.x is the lowest level of approximation for the solids (IPA) and it doesn't include any non-local part and local field effects. Moreover, you can incorporate the many-body effects in the solids using certain TDDFT kernels as well, like TDDFT-LRC (static and dynamic) and if you have solid with low electronic dielectric function and small lattice screening than the excitonic effects might play a big role even in the bulk systems, so you better solve BSE in those cases instead of RPA level of spectra. I recommend to take your system into account carefully and then choose the right theory to apply.
Best of luck,
$\begingroup$ +1. This one was unanswered for a while, thank you for attending to it! Also welcome to the site!!! We hope to see much more of you!! $\endgroup$ Jun 11, 2020 at 13:13
1$\begingroup$ Thanks, Nike, I hope I will be able to contribute to this forum in the best possible way! In fact, this question attracted me to signup! xd $\endgroup$ Jun 11, 2020 at 13:44
$\begingroup$ That's great news! How did you find this question? $\endgroup$ Jun 11, 2020 at 20:42
1$\begingroup$ Good question, the answer is, from the facebook group for the QE learners, members were invited via the link of this forum. $\endgroup$ Jun 11, 2020 at 22:50