Active Space Selection. To obtain orbitals for the active space selection scheme, we started with a restricted Hartree-Fock singlet wave function for closed-shell molecules and a restricted open-shell Hartree-Fock doublet wave function for doublet molecules, as calculated using PySCF. The molecular point group was reduced to the highest available symmetry implemented for the PySCF SA-CASSCF solver: C 2h , C 2v , C s , or D 2h . The APC-rankedorbital active-space-selection scheme starts with a set of candidate localized orbitals, ranks them by their approximated orbital entropies, and then eliminates orbitals starting from the lowest-entropy orbitals (those with the highest entropies are considered to be the most important) until the active space size reaches a predetermined maximum number of configuration state functions. We next describe the generation of candidate orbitals, then the ranking scheme, and finally the maximum-size criteria.