  We assess the utility of Hartree-Fock (HF) trial wavefunctions in performing
phaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) on the uniform
electron gas (UEG) model. The combination of ph-AFQMC with spin-restricted HF
(RHF+ph-AFQMC), was found to be highly accurate and efficient for systems
containing up to 114 electrons in 2109 orbitals, particularly for $r_s$ $\le$
2.0. Compared to spin-restricted coupled-cluster (RCC) methods, we found that
RHF+ph-AFQMC performs better than CC with singles, doubles, and triples
(RCCSDT) and similarly to or slightly worse than CC with singles, doubles,
triples, and quadruples (RCCSDTQ) for $r_s$ $\le$ 3.0 in the 14-electron UEG
model. With the 54-electron, we found RHF+ph-AFQMC to be nearly exact for $r_s$
$\le$ 2.0 and pointed out potential biases in existing benchmarks. Encouraged
by these, we performed RHF+ph-AFQMC on the 114-electron UEG model for $r_s$
$\le$ 2.0 and provided new benchmark data for future method development. We
found that the UEG models with $r_s$ = 5.0 remain to be challenging for
RHF+ph-AFQMC. Employing non-orthogonal configuration expansions or unrestricted
HF states as trial wavefunctions was also found to be ineffective in the case
of the 14-electron UEG model with $r_s$ = 5.0. We emphasize the need for a
better trial wavefunction for ph-AFQMC in simulating strongly correlated
systems. With the 54-electron and 114-electron UEG models, we stress the
potential utility of RHF+ph-AFQMC for simulating dense solids.
