The Configuration Interaction Ansatz

In order to solve Eq. (8.3), we can expand the electronic wave function 1 in a complete set of many-electron functions, , also called electronic configurations or Slater determinants. 

In the limit k oo, which implies that the number of one-particle functions also goes to 00, we obtain the exact solution of the electronic Schrodinger equation, which is the fuU configuration interaction (FCI) ansatz. A (stiU finite) expansion of the FCI wave function is only accomplishable for the smallest systems [21, 22] and further approximations to Eq. (8.6) are mandatory. 

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