Configuration interaction probabilities

A single-excitation configuration interaction (CIS) calculation is probably the most common way to obtain excited-state energies. This is because it is one of the easiest calculations to perform. 

The description of configuration interaction given for rr-electron methods is also valid for all-valence-electron methods. Recently, two papers were published in which the half-electron method was combined with a modified CNDO method (69) and the MINDO/2 method was combined with the Roothaan method (70). Appropriate semiempirical parameters and applications of all-valence-electron methods are most probably the same as those reviewed for closed-shell systems (71). 

Dunning basis sets have been optimized with atomic configuration interaction calculations and show steady improvement as the basis set quality is increased. The cc-pVQZ set is the most accurate in this category, but its size probably will preclude its use in the larger calculations... 

Let us now discuss the correlation effects on the atomic shell structure. We plot in Fig. 7 some of the described potentials for the case of the beryllium atom. The exact exchange-correlation potential v c is calculated from an accurate Cl (Configuration Interaction) density using the procedure described in [20]. The potentials Vx, and u" , are calculated within the optimized potential model (OPM) [21,40,41] and are probably very close to their exact values which can be obtained from the solution for of the OPM integral equation [21,40,41] by insertion of the exact Kohn-Sham orbitals instead of the OPM... 

The other mechanism is called the Fermi contact interaction and it produces the isotropic splittings observed in solution-phase EPR spectra. Electrons in spherically symmetric atomic orbitals (s orbitals) have finite probability in the nucleus. (Mossbauer spectroscopy is another technique that depends on this fact.) Of course, the strength of interaction will depend on the particular s orbital involved. Orbitals of lower-than-spherical symmetry, such as p or d orbitals, have zero probability at the nucleus. But an unpaired electron in such an orbital can acquire a fractional quantity of s character through hybridization or by polarization of adjacent orbitals (configuration interaction). Some simple cases are described later. 

La Paglia demonstrated convincingly how the computed electronic transition probability is very sensitive to configuration interaction which, as we have stated earlier, is only very slowly convergent. Further, the results are also sensitive to the basis set employed, and agreement between /(V) and f(R) is not satisfactory. 

In order to make allowance for the influence of electron correlation on the probability of the / -decay-induced excitation of a molecule, let us use the configuration interaction method. We will consider the configurations that take account only of the single and double electron excitations into the virtual excited orbitals. For the latter we will use the orbitals obtained by the Huzinaga-Arnau method (see above). The wave function of the ground state of the parent molecule is... 

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