Chapter 6 - One-configurational model with neglecting the non-orthogonality. The charge penetration and exchange effects

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The mutual penetration of the charge density clouds should not be identified with the overlap of the corresponding wave functions in the strict sense of the notion. There are a lot of convincing experimental data evidencing a mutual compensation of the penetration and exchange effects by the overlap effect, and more strictly by the so-called contact covalency effect. This compensation effect was probably the reason of some earlier striking successes of the point charge model. An analogous effect makes the pseudopotential in metals so weak that the almost free-electron model works very well. The effective repulsion due to the Pauli exclusion counteracts the attractive potential acting on the metal ion electrons due to their penetration into the outer ligand shells. In general, the crystal field parameters of higher orders corrected for these effects are still far from their experimental values. It thus appears that important contributions to these parameters must also arise from mechanisms, which perturb the free-ion states.