The ground-state potential curve of the beryllium dimer

Abstract

The ground-state potential curve for the beryllium dimer is calculated using different methods. In the most accurate calculation a minimum of 0.9 kcal/mole is found at a distance of 4.9 a.u. Corrections for deficiencies in the basis set lead to a predicted well depth of 2.0 kcal/mole. Three classes of methods have been used—MCSCF, CI, and MBPT methods—and calculations were performed with two different basis sets. The largest basis set has 7s, 3p, and 2d contracted Gaussian functions where the 2d functions were optimized in the molecule. Nearly complete CI calculations with this basis set recovers 97% of the valence correlation energy. The main conclusion from the calculations with the different methods and basis sets is that it is necessary to compute practically all the valence shell correlation energy in order to obtain a reliable, qualitatively correct, potential curve for Be2. The best illustration of this point is that large MCSCF calculations with 1832 configurations, obtained from a complete distribution of 4 electrons in 20 orbitals, give qualitatively wrong potential curves. This is particularly remarkable since 98.8% of the valence correlation energy obtainable with the basis set is recovered.