The ground state potential energy. Curve of Be 2: Is the MBPT approach capable of predicting it?

Abstract

The X 1Σ g + ground state potential energy curve of the beryllium dimer is studied by using different many-body perturbation theory methods. A careful examination of the dependence of the calculated data on the basis set choice and the completeness of the perturbation expansion through the given order has been carried out. It is shown that the many-body perturbation theory is capable of dealing with the weak interaction potential in the beryllium dimer provided a complete well-balanced scheme is used and the basis set is large and flexible enough. In spite of some near-degeneracy effects in Be 2: the complete fourth-order perturbation results calculated with a large (spdf) GTO basis set are very close to the corresponding variational data. The complete fourth-order treatment predicts the existence of a minimum at = 4.8 au with the dissociation energy of = 1.9 kcal/mole. Some small discrepancies between our perturbation results and very recent experimental data are within the limits of accuracy of best theoretical calculations which can be currently carried out for a system of the size of Be 2. Moreover, the existence of a long-range van der Waals minimum on the Be 2 potential energy curve is not confirmed by the present perturbation theory results.