Rayleigh-Schrodinger perturbation theory description

If the solutions (energies and wave functions P ) of the Schrodinger equation for the unperturbed system Tf(°) P = F,1,01 4/jl°l are known, and the operator form of the perturbation, Hp, can be specified, the Rayleigh-Schrodinger perturbation theory will provide a description of the perturbed system in terms of the unperturbed system. Thus, for the perturbed system, the SE is... 

IV. Many-Body Rayleigh-Schrodinger Perturbation Theory (MB-RSPT) A. Brief Description... 

Formalism of the so-called response theory is another, quite universal language for the description of the more general approach to Rayleigh-Schrodinger perturbation theory suggested above, in which the summation over excited states is effectively replaced with solving a large system of linear equa-... 

The renewal of interest in Brillouin-Wigner perturbation theory for many-body systems seen in recent years, is driven by the need to develop a robust multi-reference theory. Multi-reference formalisms are an important prerequisite for theoretical descriptions of dissociative phenomena and of many electronically excited states. Brillouin-Wigner perturbation theory is seen as a remedy to a problem which plagues multi-reference Rayleigh-Schrodinger perturbation theory the so-called intruder state problem. 

In spite of this progress, problems remain and the description of electron correlation in molecules will remain an active field of research in the years ahead. The most outstanding problem is the development of robust theoretical apparatus for handling multi-reference treatments. Methods based on Rayleigh-Schrodinger perturbation theory suffer from the so-called intruder state problem. In recent years, it has been recognized that Brillouin-Wigner perturbation theory shows promise as a robust technique for the multi-reference problem which avoids the intruder state problem. 

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