Diagrammatic perturbation theory

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Abstract
The diagrammatic many-body perturbation expansion is used to calculate potential curves for the X1∑+ state of the hydrogen fluoride molecule. Two reference hamiltonian operators are considered and for each of these the energy is evaluated through third order, [2/1] Padé approximants are constructed and upper bounds determined. The quality of the resulting potential curves is assessed by calculating spectroscopic constants. It is shown that the use of shifted denominators leads to inferior convergence properties.