Resummation of the Brillouin‐Wigner Perturbation Series.

We present a new method of resumming the Brillouin‐Wigner perturbation series with Epstein‐Nesbet partitioning. Both the nondegenerate and quasi‐degenerate cases are considered. We illustrate the accuracy of the proposed resummed Brillouin‐Wigner perturbation theory for selected atoms and small molecules, as well as a simple model system like the quartic anharmonic oscillator. For the electronic structure problems, it is found that the present theory provides essentially the same results as the corresponding configuration‐interaction calculations at the level of the fourth‐order calculations. [ABSTRACT FROM AUTHOR]