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\section{Development of exchange and correlation functionals}  Micael, %Micael,  Miguel, Xavier The central quantity of the Kohn-Sham KS  scheme of DFT is the exchange and correlation (xc) energy $E_{\rm xc}[n]$ that describes all  non-trivial many-body effects. Clearly, the exact form of this  quantity is unknown and it must be approximated in any practical 
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christened the Jacob's ladder of density functional approximations for  the xc energy. Every rung adds a dependency on another quantity,  thereby increasing the precision of the functional but also increasing  the numerical complexity and the computational time. cost.  The first three rungs of this ladder are respectively: (i)~the  local-density approximation (LDA), where the functional has a local  dependence on the density only \(n(\vec r\); only;  (ii)~the generalized-gradient approximation (GGA), that includes also a local  dependence on the gradient of the density \(\nabla n(\vec r)\); and  (iii)~the meta-GGA, that adds a local dependence on the Laplacian of  the density\(\nabla^2 n(\vec r)\)  and on the kinetic energy density  \(\tau(\vec r)\); density;  In the forth rung we have functionals that depend on the occupied Kohn-Sham orbitals, such as the exact-exchange or hybrid  functionals. Finally, the fith rung adds a dependence on the virtual  Kohn-Shame orbitals.  Support for the first three rungs and for the local part of the hybrid  functionals in Octopus is provided throught the Libxc  library\cite{Marques_2012}. library~\cite{Marques_2012}.  Libxc started as a spin-off project during the initial development of Octopus. At that point it became clear that  the task of evaluation of the xc functional was completely independent  of the main structure of code, and could therefore be transformed into  a stand-alone  library. Over the years, Libxc became more and more independent of Octopus, and is now used in a variety of DFT codes. There are  currently more than 150 xc functionals currently implemented in Libxc  that are available in Octopus, a number that is increasing steadily