The ground-state spectroscopic constants of Be2 revisited

Extensive ab initio calibration calculations combined with infinite-basis limit extrapolations lead to a ground-state dissociation energy of Be2, De=944±25 cm−1, substantially higher than the accepted experimental value. Our best computed spectroscopic observables (experimental values in parentheses) are G(1)−G(0)=223.7 (223.8), G(2)−G(1)=173.8(169±3), G(3)−G(2)=125.4(122±3), and B0=0.6086 (0.609) cm−1; revised spectroscopic constants are proposed. Multireference calculations based on a full valence CAS(4/8) reference space suffer from unbalanced description of angular correlation; for higher accuracy, the (3s,3p) orbitals should be added to the reference space. The quality of coupled-cluster results depends crucially on the description of connected triple excitations.