Density matrix renormalization group calculations were carried out without re-optimization of orbitals (in particular, SA-DMRG-CASCI) by using the state-averaged DMRG-CASCI in Block2 91 as implemented in PySCF. This will usually be called DMRG in the rest of the article (except where we wish to emphasize the distinction from SCF calculations). The maximum bond dimension (number of renormalized states) was fixed to M = 700. Calculations were done with the aug-cc-pVTZ basis as was used for the theoretical best estimates listed in the QUESTDB database. MC-PDFT calculations were carried out using a version of PySCF that incorporates the MRH code 96 now available in PySCF-Forge. 97 Grid integration was carried out for evaluation of the on-top functional with fineness grids_level = 3, as judged to be sufficient in our previous benchmark study. As in our previous study, excited-state wave functions were calculated in a state-averaged fashion averaging over the ground state and the required number of excited states (for example, to approximate the 2 1 A 2 state, we would include the ground state of symmetry 1 A 1 and two states of symmetry 1 A 2 , as in our previous work ). Final excitation energies were calculated by subtracting the ground state tPBE0 energy from the tPBE0 energy of the excited state of interest (in the example, that would be the second root in 1 A 2 symmetry).