Listing 2: Sample of the Python script used to optimize JK-fitting auxiliary basis sets for helium, neon, and argon. The full script is provided as Supplementary Material. In the JK-fitting procedure, we usually optimize for the mono-hydride of the atom, and the objective function (or loss) is the root-mean-square deviation between the full, unfitted exchange-correlation functional is used for all electronic structure calculations, with the Psi4 program (version 1.7a1) as the backend. Density fitting is turned off and the example listing also demonstrates the use of the logging functionality in BasisOpt/Python, which can be used to monitor and record progress of the optimization when used with a batch submission system.