Orbital interaction counterintuitive

Note that the third term on the right-hand side of Eq. (25) is negative (i.e. energy lowering) regardless of the sign of Hji — i.e. regardless of whether (pj combines in phase or out of phase with cj -. The case of tp > 0 describes the normal orbital interaction, and that of < 0 the counterintuitive orbital interaction. 

To understand the difference between the normal and counterintuitive orbital interactions, it is necessary to consider an alternative expression of the MO energy i.e. Ci = (pi W (pi) which, after combining with Eq. (26), leads to the expression... 

It is inherently surprising that geminal interactions are typically weaker than vicinal interactions, because the former involve orbitals that are in closer spatial proximity. The reasons for this counterintuitive distance dependence can be seen by decomposing the geminal Fock-matrix element into individual atomic hybrid contributions. 

A is a parameter that can be varied to give the correct amount of ionic character. Another way to view the valence bond picture is that the incorporation of ionic character corrects the overemphasis that the valence bond treatment places on electron correlation. The molecular orbital wavefunction underestimates electron correlation and requires methods such as configuration interaction to correct for it. Although the presence of ionic structures in species such as H2 appears counterintuitive to many chemists, such species are widely used to explain certain other phenomena such as the ortho/para or meta directing properties of substituted benzene compounds under electrophilic attack. Moverover, it has been shown that the ionic structures correspond to the deformation of the atomic orbitals when they are involved in chemical bonds. 

Thus, the strength of the interaction depends on the overlap S, and the other proportionality constant k - which has nothing to do with reciprocal space - is chosen as 1.75 it may also be an adjustable parameter. There is also a weighted, improved form of the Wolfsberg-Helmholz equation that helps to suppress so-called counterintuitive orbital mixing [113], a weakness of EH theory and also other minimum basis-set methods. 

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