  The chromium dimer (Cr2) presents an outstanding challenge for many-body
electronic structure methods. Its complicated nature of binding, with a formal
sextuple bond and an unusual potential energy curve, is emblematic of the
competing tendencies and delicate balance found in many strongly correlated
materials. We present a near-exact calculation of the potential energy curve
(PEC) and ground state properties of Cr2, using the auxiliary-field quantum
Monte Carlo (AFQMC) method. Unconstrained, exact AFQMC calculations are first
carried out for a medium-sized but realistic basis set. Elimination of the
remaining finite-basis errors and extrapolation to the complete basis set (CBS)
limit is then achieved with a combination of phaseless and exact AFQMC
calculations. Final results for the PEC and spectroscopic constants are in
excellent agreement with experiment.
