DFT, being an exact theory for ground state properties, often works well to compute structural properties but doesn't provide accurate excited state properties such as band gaps. A more rigorous framework for the description of excited states is provided by MBPT based on Green's functions and the concept of quasiparticles. The quasiparticle energies are the energies for adding or subtracting an electron from a many-electron system. Using the same Green's function-based MBPT framework, neutral excitations, which can be directly compared to experimental optical absorption spectra, can also be calculated from the solution of the Bethe-Salpeter equation (BSE). The BSE represents one of the most accurate yet computationally tractable approaches for the ab initio study of neutral excitations in crystalline systems by including the attractive interaction between electrons and holes (excitonic effects) using two-particle Green's function thus going beyond the single-particle picture of DFT within random-phase approximation (RPA).