Inclusion of ISCI and FISCI correlations

After this necessary excursion into the broad field of electron-electron interactions as described in diagrammatic language and by certain approximations, the effects of electron correlations beyond the RRPA approach can be summarized by using the following rules (neglecting the photoelectron s self-energy diagram) ISCI and FISCI must be taken explicitly into account, and the theoretical ionization threshold has to be adapted to the experimental value. 

The correlated wavefunction which incorporates ISCI follows in analogy to equ. (1.25a) as an expansion into independent-particle wavefunctions for the ground state and contributions from virtual two-electron excitations  

The mixing coefficients can be determined by a multiconfigurational Dirac or Hartree-Fock procedure (MCDF, MCHF). In the present case, however, numerical values are not of interest, only the fact that A0 is smaller than unity due to the presence of virtual excitations in the normalized correlated wavefunction. 

Similarly, FISCI can be incorporated into the wavefunction of the final ionic state  

The coefficient a0 belongs to the process under consideration if FISCI effects are not dramatic it will have the largest value. As well as the characteristic 2p-2h excitations attached to the coefficients f, terms of lp-lh excitations have also 

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