Hiiekel method

Hoffmann introdueed the so-ealled extended Hiiekel method. He found that a value for K= 1.75 gave optimal results when using Slater-type orbitals as a basis (and for ealeulating the Sg,v)- The diagonal h elements are given, as in the eonventional Hiiekel method, in terms of valenee-state IP s and EA s. Cusaehs later proposed a variant of this parameterization of the off-diagonal elements ... 

In the Hiickel or extended Hiiekel methods no explicit reference is made to electron-electron interactions although such contributions are absorbed into the V potential, and... 

Hoffmann introdueed the so-ealled extended Hiiekel method. He found that a value for K= 1.75 gave optimal results when using Slater-type orbitals as a basis (and for ealeulating the... 

As presented, the Roothaan SCF proeess is earried out in a fully ab initio manner in that all one- and two-eleetron integrals are eomputed in terms of the speeified basis set no experimental data or other input is employed. As deseribed in Appendix F, it is possible to introduee approximations to the eoulomb and exehange integrals entering into the Foek matrix elements that permit many of the requisite Fj, y elements to be evaluated in terms of experimental data or in terms of a small set of fundamental orbital-level eoulomb interaetion integrals that ean be eomputed in an ab initio manner. This approaeh forms the basis of so-ealled semi-empirieal methods. Appendix F provides the reader with a brief introduetion to sueh approaehes to the eleetronie strueture problem and deals in some detail with the well known Hiiekel and CNDO- level approximations. 

Recent calculations based on the extended Hiiekel (EHT) molecular orbital method for the 1-cyclopropylvinyl cation indicate that the ion is more stable in the linear bisected conformation (Kelsey and Bergman 1971). 

Simple Hiiekel calculations on benzene, in eontrast, place all the n electrons in bonding MOs. The re-electron energy of benzene is ealeulated by summing the energies of the six n electrons, which is 6a + 8jS, lower by 2/1 than the value of 6a + 6jS for three isolated double bonds. Thus, the HMO method predicts a special stabilization for benzene. 

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