where, 𝑇 [𝜌(𝒓)] and E ee [ρ(𝐫)] are the exact kinetic energy and electron-electron interaction energy, respectively. Since the E XC [ρ(𝐫)] term is not fully known, therefore, an approximate functional depending upon the electron density has been used to describe E XC [ρ(𝐫)]. Till now, several approximated exchange-correlation functionals have been proposed. These functionals can be categorised according to Jacob's ladder scheme (Figure ) which was proposed by Perdew 65,66 . In Jacob's ladder scheme, each rung of the ladder contains a group of a different functional set, which have been categorised on basis of their complexity. This ladder provides a direction from the Hartree approximation on "earth" to the exact exchange-correlation functional in "heaven". Furthermore, the exchange correlational functional can be categorized into empirical and non-empirical, as mentioned in Figure . A number of possible approximations, e.g., local density functional (LDA, generalised gradient approximation (GGA) etc. and a few of the exchange-correlation functional which are generally used in the DFT calculations, are discussed in the following section.