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 Cr-2 molecule is an outstanding example, which we previously treated with highly accurate auxiliary-field quantum Monte Carlo (AFQMC) calculations [W. Purwanto et al., J. Chem. Phys. 142, 064302 (2015)]. Somewhat surprisingly, computational description of the isoelectronic Mo-2 dimer has also, to date, been scattered and less than satisfactory. We present high-level theoretical benchmarks of the Mo-2 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 Cr-2 and Mo-2. Published by AIP Publishing.
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