Counterintuitive orbital mixing

The spartan reliance of the EHM on empirical parameters helps to make it relatively easy (in the right hands) to interpret its results, which depend, in the last analysis, only on geometry (which affects overlap integrals) and ionization energies. With a strong dose of chemical intuition this has enabled the method to yield powerful insights, such as counterintuitive orbital mixing [65], and the very powerful Woodward-Hoffmann rules [38]. 

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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