Nearly-exact calculation of chromium dimer binding with auxiliary-field quantum Monte Carlo

The binding of the strongly correlated Cr$_2$ molecule has long resisted accurate theoretical description, and Cr$_2$ has become a landmark test for many-body computational methods. We first performed exact auxiliary-field quantum Monte Carlo (AFQMC) calculations using a moderately-sized basis set. In parallel, phaseless AFQMC\footnote{% Zhang and Krakauer, \textit{Phys. Rev. Lett.} \textbf{90}, 136401 (2003)} calculations were carried out using the same and larger basis sets to remove the finite-basis errors from the exact AFQMC calculations. Results on Cr$_2$ ground-state properties, including binding energy, equilibrium distance, and vibrational frequency, are in excellent agreement with experiment.