Configuration-interaction theory truncated expansions

This requirement is satisfied by two of the methods most frequently used to calculate correlation effects Many body perturbation theory (MBPT) and coupled cluster theory (CC). In principle, configuration interaction (Cl) is also size consistent, but only if it is not truncated at a certain excitation level. Since untruncated Cl expansions become untractably large even for medium-size systems. Cl is not a method that can be used for the calculation of molecule/ surface interactions. 

In practice, the different expansions for the correlation energy afforded by coupled cluster theory, configuration interaction and perturbation theory have to be truncated in order to render computations tractable. The different methods differ only in the way in which this truncation is carried out. However,... 

In Chapter 11, we treat configuration-interaction (Cl) theory, concentrating on the full Cl wave function and certain classes of truncated Cl wave functions. The simplicity of the Cl model allows for efficient methods of optimization, as discussed in this chapter. However, we also consider the chief shortcomings of the Cl method - namely, the lack of compactness in the description and the loss of size-extensivity that occurs upon truncation of the Cl expansion. 

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