Figure : Same as Figure but for H 4 H 5 is a doublet for which contributions beyond perturbative triples take on importance. As shown in Figure , UCCSD(T) obtains sub-mHa accuracy vs FCI. Interestingly, GCCSD(T) is slightly less accurate than UCCSD(T); while the GHF ⟨ Ŝ2 ⟩ is lower than that of UHF, the presumed breaking of Ŝz symmetry at the GHF level results in a consistent (albeit very slight) underestimation of the correlation energy along the dissociation coordinate. In the region of maximum spin-contamination at the GHF level, both variants of ph-AFQMC perform very accurately. Evidently, the imaginary time propagator is effectively damping contributions from excited eigenstates of incorrect S 2 and S z ; in solving the Coupled Cluster equations, no such benefit arises. At a bond length of 1.4Å, we find (as we did for H 3 ) that the ph-AFQMC-G outperforms ph-AFQMC-U by roughly 3 mHa. The former is approximately 2.5 mHa above the FCI value, and ∼0.5 mHa above the maximum error of GCCSD(T). As we proceed to H 7 (Figure ), we encounter the first odd-membered hydrogen ring system in which ph-AFQMC-G outperforms even UCCSD(T) (with a slight difference in maximum error of 0.5 mHa). The maximum error of ph-AFQMC-G is a modest 1.5 mHa near the FCI equilibrium bond length of ∼1.2 Å. As before, GCCSD(T) makes larger errors than UCCSD(T), likely due to the S z contamination of the GHF reference, now having a maximum error from FCI of roughly 4.5 mHa (the worst among the four studied models).