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.