Comparison within the algebraic approximation of many-body perturbation theory and configuration interaction for the beryllium dimer

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Abstract
The ground state potential energy curve for the beryllium dimer is calculated using non-degenerate many-body perturbation theory and the multi-configuration self-consistent-field/configuration interaction method. Quasi-degeneracy in this system makes it useful in exploring the limitation of the applicability of the non-degenerate formulation of diagrammatic many-body perturbation theory. Both methods are applied within the algebraic approximation defined by a contracted gaussian basis set of triple zeta quality. It is shown that non-degenerate perturbation theory can lead to a potential energy curve which is in close agreement with the configuration interaction curve when taken to third order in the energy and [2/1] Padé approximants constructed.