Configuration interaction definition

Figure 4. One-dimensional MNDO isomerization pathways for tetra-methyl tetrahedrane and its radical cation. For the definition of the coordinate a cf. ( ) Cl denotes calculations including limited configuration interaction (see text).

That does not mean that a valid semiempirical parametrization based on the HFR MO LCAO scheme cannot be built for a certain narrow class of compounds or even for a specific purpose. It is done for example in [86] even for iron(II) porphyrins. But in a more general case there is no way to arrive at any definite conclusion [118] about the validity of a semi-empirical parametrization relying on the HFR approximation. On the other hand we have to mention that the semiempirical method ZINDO/1 [119] which allows for some true correlation by taking into account the configuration interaction may be considered a prospective setting for further parametrization, provided the HFR solution required by this method as a zero approximation can be obtained. This will be discussed in more detail later. 

The coupled cluster and configuration interaction equations presented thus far in this chapter have implicitly used spin-dependent molecular orbitals for their definitions of determinants, integrals, and wavefunction amplitudes. 

In addition to MP2, MP3, and MP4 calculations, CCSD(T), CASSCF, FOCI (First-Order Configuration Interaction), and sometimes SOCI (Second-Order Configuration Interaction) approaches have been used to ensure the convergence of the results. The complete definitions of the variational spaces used are given in [61,62,63]. Electronically-excited states have been obtained by means of the MC/P method, recently developed in our group [64,65] it couples a variational treatment to deal with the nondynamic correlation effects and a perturbation treatment to account for the dynamic correlation effects as well as the non-dynamic effects not treated at the variational level becanse of their limited contributions to the phenomena investigated. All electronic transitions reported here are vertical transition energies. 

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