State-specific nondynamical correlation

The complementarity of variational- and perturbative-type approaches, specifically of Cl and CC methods, should now be obvious While the former ones can simultaneously handle a multitude of states of an arbitrary spin multiplicity, accounting well for nondynamic correlation in cases of quasidegeneracy, they are not size-extensive and are unable to properly describe dynamic correlation effects unless excessively large dimensions can be handled or afforded. On the other hand, CC approaches are size-extensive at any level of truncation and very efficiently account for dynamic correlation, yet encounter serious difficulties in the presence of significant nondynamic correlation effects. In view of this complementarity, a conjoint treatment, if at all feasible, would be highly desirable. 

In Chapter 2, the author describes state- and property-specific quantum chemistry including comments on certain aspects of the separation of electron correlation into its dynamical and nondynamical parts. The account contains also a historic appraisal of multiconfigurational schemes comparing Brueckner-Goldstone many-body perturbation theory, MBPT,... 

Multiple PESs may be of simultaneous interest based not only on physical reasons, as emphasized in the previous paragraph, but also for mathematical or computational reasons. Consider the basic paradigm of state-selective methods the nondynamical electron correlation for a specific state is calculated within a model space and then the dynamical electron correlation is calculated. The implicit assumption is that the zero-order model space many-electron basis functions (MEBFs) (e.g., MCSCF functions and MCSCF complementary space... 

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