Mixing with excited configurations

Apart from the interest in the properties of the excited configurations themselves, their influence on the f-electrons has been studied extensively. As mentioned in the beginning of this section, mixing with excited configurations is responsible for the observed intensities of f-f transitions. This feature will be discussed in sect. 5.3.1. The influence on the center of gravity of the multiplets is also taken into account in most studies via the two-body and three-body configuration interaction parameters. 

However, it is wise to be careful. If the SOMO (singly occupied MO) lies close in energy to other orbitals, the radical must be described as a combination of several electronic configurations (by configuration interaction57). Thus, the ground state (a) may be affected by mixing with excited states, such as ... 

The ground state electron configuration of SiC is (5a)2 (6a)2 (7a)1 (2 r)3 so that the hyperftne constants will reflect the electron distribution in both a and n molecular orbitals. The hyperftne constants a and d are determined mainly by the unpaired n electron, the Fermi contact constant hv depends mainly on the a electron, whilst c is sensitive to both a and tt electrons. In fact the Fermi contact constant also depends, as usual, upon configurational mixing with excited electronic states, which makes... 

In an electron donor-acceptor system (D-A) in which an electron donor (D) and an electron acceptor (A) are separated by a carbon chain, the exciplex state may be considered as a resonance hybrid of the electron transfer configuration (D A" ) mixed with the locally excited configuration (D A) or (DA ) [211]. 

Electron configuration of Bp" is (6s) (6p) yielding a Pip ground state and a crystal field split Pap excited state (Hamstra et al. 1994). Because the emission is a 6p inter-configurational transition Pap- Pip. which is confirmed by the yellow excitation band presence, it is formally parity forbidden. Since the uneven crystal-field terms mix with the (65) (75) Si/2 and the Pap and Pip states, the parity selection rule becomes partly lifted. The excitation transition -Pl/2- S 1/2 is the allowed one and it demands photons with higher energy. 

Excited-state energies and wave functions are automatically obtained from Cl calculations. However, the quality of the wave functions is more difficult to achieve. The equivalent of the HF description for the ground state requires an all-singles Cl (SCI). Singly excited configurations do not mix with the HF determinant, that is,... 

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