  The three-band Hubbard model is a fundamental model for understanding
properties of the Copper-Oxygen planes in cuprate superconductors. We use
cutting-edge auxiliary-field quantum Monte Carlo (AFQMC) methods to investigate
ground state properties of the model in the parent compound. Large supercells
combined with twist averaged boundary conditions are studied to reliably reach
the thermodynamic limit. Benchmark quality results are obtained on the magnetic
correlations and charge gap. A key parameter of this model is the
charge-transfer energy $\Delta$ between the Oxygen p and the Copper d orbitals,
which appears to vary significantly across different families of cuprates and
whose ab initio determination is subtle. We show that the system undergoes a
quantum phase transition from an antiferromagnetic insulator to a paramagnetic
metal as $\Delta$ is lowered to 3 eV.
