The choice of the exchange-correlation functional (often simply called density functional, DF) determines the accuracy of DFT computations to a certain extent. There is no systematic way to improve a functional but depending on the "ingredients" used for their development, several classes of functionals can be distinguished (Fig. .). These classes have been put in a hierarchy which is most often illustrated by referring to the biblical picture of "Jacob's ladder" (in this context climbing from the "earth" of HF to the "heaven" of the exact DF; in the original version the top is given as "chemical accuracy" which however is an ambiguous term) which results in the functionals being classified into "rungs" of this ladder. The simplest DFs belong to the rung of local density approximation (LDA). They only depend on the electron density at a certain point and the corresponding energy 𝜀 𝑋𝐶 (eq. 2.4).