The Use of Multireference Functions in Perturbation Theory

There has been a revival of interest in the Brillouin-Wigner perturbation theory since it is seen as a possible remedy to the intruder state problem. As 

Wenzel and Steiner197 have pointed out ... the reference energy in Brillouin-Wigner perturbation theory is the fully dressed energy. .. This feature guarantees the existence of a natural gap and thereby rapid convergence of the perturbation 

The use of Brillouin-Wigner perturbation theory in describing the many-body problem has been considered recently by Hubac and Wilson.198 These authors use the identity 

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