Localization and Correlation

The basic items of electron pair theories are reviewed, in particular the separated-, independent-, and coupled-electron pair approaches are outlined. The results of these approaches are not invariant under unitary transformations among the strongly occupied orbitals, which leave the Hartree- Fock wave function unchanged. The choice of localized (equivalent) orbitals has some advantages compared to that of delocalized (canonical) orbitals. In the localized representation certain pair correlation energies are transferable between related molecules. The pair coupling terms between localized pairs are usually small in absolute value and mostly positive whereas those between canonical pairs are large in absolute value, have either sign and cancel to a large extent. The relations between localization, correlation, and chemical bonding are discussed.

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