On the Elusive Twelfth Vibrational State of Beryllium Dimer

The beryllium dimer has puzzled chemists for roughly 80 years on account of its unexpectedly strong bonding interaction between two nominally closed-shell atoms. Recent spectroscopic measurements characterized the molecule's ground electronic state with sufficient resolution to distinguish 11 vibrational levels; the possibility that a twelfth level lay just below the dissociation threshold remained unresolved. Here we present a potential function, based on ab initio calculations at the full configuration interaction level, that definitively supports the presence of this twelfth vibrational state. Morphed versions of this potential, fitted to experimental data, closely reproduce the observed spectra to within 0.1 cm(-1), bolstering the strength of the assignment.