Single parameter representation for the BLYP (upper panel) and GAM (lower panel) exchange–correlation functionals as a 3D function (left plots) of the Wigner-Seitz radius \(r_s\), eq. \ref{eqn:8} and reduced density variables \(u\), eq. \ref{eqn:4}. The right plots represent slices at constant \(r_s=2.5\). For both functionals, the black dots are the decoded points, the orange solid curve is the original enhancement factor as obtained directly from LibXC, and the dashed red curve is the result of the decoding of the single parameter and the interpolation via univariate cubic spline. Results are obtained on a grid of  \(20\times20\)  points and \(\beta=12\). A Jupyter notebook to encode every GGA or NGA exchange–correlation functional in LibXC, as well as to reproduce the plots and to calculate the encoding and interpolation errors is associated with the Figure.