Method Timing. All converged CASSCF wave functions (orbitals and CI vectors) were saved to disk at the end of the calculation. Timings for tPBE and NEVPT2 calculations were achieved by loading in the converged CASSCF wave functions, computing the relevant quantity (the tPBE nonclassical energy or the NEVPT2 perturbative correction) and then saving the results. The amount of resources requested for each calculation was determined by an empirically derived formula dependent on the number of aug-cc-pVTZ basis functions in the underlying molecule (SI Section 8.2), and so timings between the tPBE and NEVPT2 implementations available in PySCF can be fairly compared (although we note that methodologies can always be further optimized).