Full configuration interaction calculations on Be₂

Valence full configuration interaction calculations are reported for the beryllium dimer using an ANO-type contraction and including up to g functions. It is shown that very large contracted basis sets are required in order to obtain a good value of the dissociation energy. The electronic energy curve is repulsive in the bond region if only s and p orbitals are used, and f and g orbitals account for about one half of the total dissociation energy. It is also shown that the use of molecular orbitals specifically optimized for the dimer can lead to a substantial improvement of bond distance and dissociation energy values.