Accurate ab initio structure and vibration–rotation dynamics of the beryllium dimer

The accurate potential energy function of the beryllium dimer in its ground electronic state has been obtained. The valence-electron correlation energy was calculated using the coupled-cluster approach, up to the CCSDTQ level of approximation (equivalent in this case to the full-configuration-interaction level of theory) with the augmented correlation-consistent basis sets as large as aug-cc-pV10Z. The core-electron correlation effects were estimated up to the CCSDTQ level using the core-valence basis sets as large as aug-cc-pCV7Z. The scalar relativistic, adiabatic, and non-adiabatic effects on spectroscopic constants of Be2 were also discussed. In particular, the adiabatic binding energy De of Be2 was predicted without invoking any extrapolation procedures to be 935 cm-1, with an uncertainty of ±2 cm-1. The vibrational term values and the effective rotational constants of Be2 were predicted to be within the corresponding experimental error bars.