After obtaining the Khon-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 such interaction to be relevant for molecules (small ones, at least). What about periodic crystal, should I be concerned about it?