  The use of an approximate reference state wave function |Phi_r> in electronic
many-body methods can break the spin symmetry of Born-Oppenheimer
spin-independent Hamiltonians. This can result in significant errors,
especially when bonds are stretched or broken. A simple spin-projection method
is introduced for auxiliary-field quantum Monte Carlo (AFQMC) calculations,
which yields spin-contamination-free results, even with a spin-contaminated
|Phi_r>. The method is applied to the difficult F2 molecule, which is unbound
within unrestricted Hartree-Fock (UHF). With a UHF |Phi_r>, spin contamination
causes large systematic errors and long equilibration times in AFQMC in the
intermediate, bond-breaking region. The spin-projection method eliminates these
problems, and delivers an accurate potential energy curve from equilibrium to
the dissociation limit using the UHF |Phi_r>. Realistic potential energy curves
are obtained with a cc-pVQZ basis. The calculated spectroscopic constants are
in excellent agreement with experiment.
