Full configuration interaction efficiency

Complete Cl, or full Cl, is configuration interaction with a configuration list which includes all possible configurations of proper spin and space symmetry in the chosen orbital space. As has been mentioned previously, the number of configurations in complete Cl will depend in an n-factorial way on the number of electrons and the number of orbitals and it will therefore quickly become too large to be handled. This method is therefore not very well suited as a standard model to solve quantum chemical problems. There are, however, two situations where an efficient complete Cl method is useful to have. The first of these is in connection with the CASSCF method which has been described in another chapter. The other is in connection with bench mark tests. Since any other Cl method selects configurations after some principle, a comparison to complete Cl is the way to check these principles out. We will therefore in this section briefly outline the main steps in the complete Cl method as it is carried out today. 

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. 

---