For the last thirty years, the structural, magnetic and optoelectronic properties of bulk crystals and molecules have been studied with computational tools based on the density functional theory (DFT), a successful quantum mechanical approach to the properties of solids, which has contributed, hitherto, invaluable scientific knowledge. The reason for this success relies on the Hohenberg-Kohn theorems and the Kohn-Sham differential equation, which provide a way to systematically map the many-body problem of many interacting electrons in a system, computationally very hard to solve with methods such as Hartree-Fock or configuration interaction, onto a simpler single-body problem. We stress that, although DFT looks formally like a single-particle theory, many-body effects are included via the exchange-correlation functional. The Hohenberg-Kohn theorem states that, given a ground-state electronic density n, it is possible to calculate the ground-state many-body wave function and all ground-state observables. Among the available computer codes to perform DFT calculations, we employ "VASP" and “Quantum Espresso”, which uses plane wave expansions to build up the basis sets, and also “CP2K”, which mixes plane waves and Gaussian functions and is suitable for large systems with thousands of atoms. In practice, these codes deal only with the valence electrons, since the core electrons plus the atomic nucleii are described by means of pseudopotentials; these effective potentials are commonly generated in the generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA).

In particular, perovskite bulk compounds and nanostructures have been also investigated with DFT tools for the last years. In our group, we have performed DFT computations for many different purposes: for instance, to obtain the relative stability of the crystal phases of FAPbI₃ in the presence of PbS [1] and PbO quantum dots; to compute band offsets in heterojunctions before and after photoexcitation [2]; to assess the role of secondary ammonium cations in the long-term stability of 2D/3D perovskites [3]; to extract structural and optical properties of the bulk compound CsBr when doped with interstitial Sn atoms [4]; to study the stability of FAPb_1-xSr_xI₃, where Sr is a less toxic metal than Pb [5]; to calculate the formation energies of inorganic nanocrystals of CsPbI₃ with tocopherol ligands on surface; and to investigate perovskite heterostructures composed of either MAPbI₃ or FAPbI₃ matrices with embedded PbS nanoparticles.

Based on all these results, in this talk I will present a picture of DFT computations for perovskite materials of interest for optoelectronic applications such as LEDs and solar cells.