In Jacob’s ladder, the different approximations for the exchange and correlation functional are described as rungs. The base level is the Hartree-Fock approach, then comes the inclusion of the electron density, next the gradient, next the laplacian. After that a mix of exact exchange with the gradient of the homogeneous electron gas. After that, the unoccupied Kohn-Sham orbitals are also considered. Each rung improves (in some cases) the description of the electron density in the materials (or molecules), at the cost of computational effort and theoretical complexity in the description. The heaven of chemical accuracy is the holy grail of solid-state ab-initio calculations.