  Extensive ab initio calibration calculations combined with extrapolations
towards the infinite-basis limit lead to a ground-state dissociation energy of
Be_2, D_e=944 \pm 25 1/cm, 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 \pm 3), G(3)-G(2)=125.4 (122 \pm 3), and
B_0=0.6086 (0.609) 1/cm; revised spectroscopic constants are proposed.
Multireference calculations based on a full valence CAS(4/8) reference suffer
from an unbalanced description of angular correlation; for the utmost accuracy,
a CAS(4/16) reference including the $(3s,3p)$ orbitals is required, while for
less accurate work a CAS(4/4) reference is recommended. 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.

Chemical Physics;Atomic and Molecular Clusters