The ground-state spectroscopic constants of Be2 revisited

Extensive ab initio calibration calculations combined with extrapolations towards the infinite-basis limit lead to a ground-state dissociation energy of Be2, De=944±25 cm−1, substantially higher than the accepted experimental value, and confirming recent theoretical findings. Our best computed spectroscopic observables (expt. values in parameters) 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 an unbalanced description of angular correlation; for the utmost accuracy, the (3s, 3p) orbitals should be added to the reference space. The quality of computed coupled cluster results depends crucially on the description of connected triple excitations; the CC5SD(T) method yields unusually good results because of an error compensation.