  Chemical accuracy is difficult to achieve for systems with transition metal
atoms. Third row transition metal atoms are particularly challenging due to
strong electron-electron correlation in localized d-orbitals. The Cr2 molecule
is an outstanding example, which we previously treated with highly accurate
auxiliary-field quantum Monte Carlo (AFQMC) calculations [Purwanto et al., J.
Chem. Phys. 142, 064302 (2015)]. Somewhat surprisingly, computational
description of the isoelectronic Mo2 dimer has also, to date, been scattered
and less than satisfactory. We present high-level theoretical benchmarks of the
Mo2 singlet ground state ($X ^1\Sigma_g^+$) and first triplet excited state ($a
^3\Sigma_u^+$), using the phaseless AFQMC calculations. Extrapolation to the
complete basis set limit is performed. Excellent agreement with experimental
spectroscopic constants is obtained. We also present a comparison of the
correlation effects in Cr2 and Mo2.
