2. Fitting elephants: One-parameter fit of exchange–correlation functionals.

This section briefly discusses the application of Piantadosi’s encoding procedure \cite{piantadosi_one_2018}  to describe any local xc functional with a single real-valued parameter \(\alpha\in[0,\ 1]\). The simplest case of a generalized gradient approximation (GGA) exchange functional is illustrated first, since it just requires a straightforward mono-dimensional fit. The more complex case of meta-GGA exchange functionals  and GGA exchange–correlation functionals are also presented next. Jupyter notebooks with the code developed for each of these cases accompany the electronic version of this article and are available for download using the interactive features of this special issue and on the Author's github page. These programs allow to obtain single-parameter representations for the majority of the more than 300 xc functionals that are included in the LibXC library \cite{marques_computer_2012,lehtola_recent_2018}.