To perform a ph-AFQMC calculation one needs to define some trial |Ψ T ⟩ and some initial set of walkers {|φ i (0)⟩}. For the UHF and GHF trials we simply initialize all the walkers to be the same as the trial. For the ph-AFQMC calculations using truncated CASSCF trials (see Supporting Information for details), the walkers were initialized stochastically according to the CI weight distribution in the CASSCF wavefunction.