Configuration interaction ionisation

In order to obtain increased accuracy it is necessary to include atomic orbitals higher than Is and such a configuration interaction calculation by McLean, Weiss and Yoshimine [26] which employed atomic Is, 2s and 2p atomic orbitals gave a binding energy of 4.55 eV Later calculations, particularly by Kolos and Wolniewicz [27], produced energies for dissociation and ionisation of H2 which were within 10 4 eV or less of the experimental values. 

Fig. 11.12. The experimental and theoretical branching ratios for the 1000 eV ionisation of lead to the 6ps/2 and 6pi/2 states of Pb+, plotted against recoil momentum p (Frost et al, 1986). The calculations with target-state correlations in the plane wave impulse approximation are indicated by MCDF, multiconfiguration Dirac—Fock EAL, extended average level OL, optimal level. Cl indicates ion-state configuration interaction.

Geometric and electronic properties are obviously mutually interdependent. These also influence, and are influenced by, the interaction of chemical entities with their environment (e.g., solvent). A number of molecular properties which are accessible by experiment result from, or are markedly influenced by, interactions with the environment (e.g., solvation, ionisation, partitioning, reactivity). For these reasons, the concept of chemical structure must be extended to include interaction with the environment. Table 1 summarizes the above discussion and may help broaden the intuitive grasp of the concept of chemical structure. Table 1 is also useful in that it allows a delineation of the matters to be discussed in this chapter. As indicated by the title, we will consider molecules at the geometric levels of modellization, either as rigid (configurational aspects) or as flexible geometric objects (conformational aspects). Broader conceptual levels (electronic features, interaction with the environment) lie outside the scope of this chapter and will be considered only occasionally. 

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