In H 3 (Figure ) the coupled cluster methods are more or less exact (errors on the scale of 0.1mHa), as there are only 3 electrons and CCSDT is sufficient to capture all the electron correlation. At short distances there is no UHF to GHF instability; as such the U-and G-QMC results are identical (to within stochastic noise). Beyond this point we enter a region with a UHF to GHF instability. Note that the maximum error of ph-AFQMC-G is approximately half that of ph-AFQMC-U relative to FCI. This is a direct manifestation of the terms in θ