Eliminating spin contamination in auxiliary-field quantum Monte Carlo:: Realistic potential energy curve of F2

The use of an approximate reference state wave function vertical bar 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 vertical bar Phi(r)>. The method is applied to the difficult F-2 molecule, which is unbound within unrestricted Hartree-Fock (UHF). With a UHF vertical bar 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 vertical bar Phi(r)>. Realistic potential energy curves are obtained with a cc-pVQZ basis. The calculated spectroscopic constants are in excellent agreement with experiment. (c) 2008 American Institute of Physics.