Extended Huckel method description

Throughout the studies discussed above, there has been a close interplay between structural measurements and development of theoretical models of molecular electronic structure. Lipscomb s contributions have included topological description of the boron hydrides and fundamental theory. Several coworkers contributed in this area, but most notable is the work with Roald Hoffmann, which changed the way chemists approach the theory of molecules of interesting complexity (see, e.g., the comments in (2(5)). The extended Huckel method was developed in the Lipscomb s group by several people, including especially L. L. Lohr, Jr., and Roald Hoffman (see Roald s recollections in Current Contents Citation Classic, May 8, 1989). Although the method probably contributed more to chemistry than any other method imtil very recently, Lipscomb recalled decades after he introduced it that this method received intense criticism, even denouncement. More exact theory led to the first correct calculation of the rotational barrier in ethane (with R. M. Pitzer in 1963). 

Certainly, Eq. (6.89) is only a specific case of Eq. (6.4), and no EUE effects are possible at this level of description. It would be important to extend the Huckel model in order to somehow account for electron correlation effects without oversimplifying the model. The approximation of this kind was given in [108] and applied to EUE problems in [13]. The most important expressions of this work are reproduced here (see cit. loc. for the argumentation and precursors of the model). The model was referred as to the quasi-correlated tight-binding (QCTB) method. Within QCTB, we construct the effective Hamiltonians matrices... 

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