Brillouin-Wigner perturbation theory BWPT

In Section 14.1, we developed Rayleigh-Schrodingerperturbation theory (RSPT), where the enei gy and the wave-function corrections are obtained in a noniterative form. In this exercise, we develop Brillouin—Wigner perturbation theory (BWPT), where the corrections are generated iteratively. 

BWPT = Brillouin-Wigner perturbation theory EN = Epstein-Nesbet FCI = full Cl MBPT = many-body perturbation theory MRS PT = multireference state perturbation theory PT = perturbation theory RSPT = Rayleigh-SchrSdinger perturbation theory SRS PT = single-reference state perturbation theory. 

MR-BWPT multi-reference Brillouin-Wigner perturbation theory MR-RSPT multi-reference Rayleigh-Schrodinger perturbation theory... 

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