Extended Hiickel treatment

All this suggests a further simplification, which has proved to be eminently successful in many cases. It is known that independent electron treatments, such as the Hiickel (HMO) treatment2 or the extended Hiickel treatment (EHT)172, which do not take the electron-electron interaction explicitly into account, yield—by and large—orbitals derived from sophisticated SCF calculations. In particular, the HMO and ETH molecular orbitals reflect faithfully the symmetry and nodal properties of their counterparts obtained from SCF treatments. 

Hiickel and extended Hiickel treatments that consider many fundamental ideas of orbitals and bonding but neglect electron repulsions in molecules. Treats systems up to 200 atoms. 

Anderson modification of extended Hiickel treatment, 34 136 -MO, 37 151-153 Anderson structure heteropolyacids, 41 123 Andrews-Beams turbine spinner, 33 209 Aniline... 

ASED, see Anderson modification of extended Hiickel treatment Ash, 27 317... 

Effective medium theory, 37 154 Eggshell catalysts, 39 231 EH method, 37 153 EHT, see Extended Hiickel treatment Eigenberger model, oscillatory reactions, 39 80-81, 83... 

Exothermic reaction parameters of, 27 63 temperature oscillations, 27 65-67 Explosion, petovskite preparation, 36 250 Extended Hiickel treatment, 34 136, 147, 154, 156, 166, 173... 

The same theoretical methods have been applied to ethylidyne absorbed in the Rh(100) surface, with the conclusion, as experimentally found, that this species will be stable on the 4-fold site (237). However, the extended-Hiickel treatment, as well as the atom-superposition and electron-delocalization molecular orbital (ASED-MO) method, predict that the CC bond length will be shorter, and hence the vCC mode higher in wavenumber on the 4-fold site, whereas experimentally... 

Semiempirical quantum-chemical methods can be subdivided into two groups. The first one covers so-called simple MO LCAO methods, of which extended Hiickel treatment (EHT) (7) and its modification by Anderson (ASED) (5), additionally taking into account core repulsion, are most widely used in chemisorption computations. Anderson s improvement of EHT was aimed at obtaining more reliable values of the total energy and at gaining an opportunity to optimize the geometry of chemisorption structures. The method was mainly used in calculations of chemisorption and catalysis on metals. Its validity for oxide systems, with their rather highly ionic bonds, is formally less justified. 

The coefficients of the atomic orbitals in the retype molecular orbitals of the extended Hiickel treatment are reasonably close to those of both the Hiickel and Pople-SCF 7r-electron calculations on NF2, cis- and ran -N2F2. The deviations tend to increase with increasing orbital energy. 

In this research we have found that for general descriptions of bonding in N, F compounds an extended Hiickel treatment leads to results consistent with the properties and behavior of known N, F compounds. Furthermore, certain other, as yet uninterpreted properties of these systems can be gleaned from the calculational results. 

A more ambitious Extended Hiickel Treatment has been developed by Pyykko and Lohr [76] allowing the inclusion of relativistic effects with refined orbital exponents. In its most recent form the method, known as REX, does not rely on empirical parameters but rather derives its parameterisation from ab initio atomic calculations, and allows for charge-consistency [51], The importance of charge-consistency in EHT calculations was pointed out long ago... 

In the simple Hiickel theory of planar hydrocarbons, each tt MO contains contributions from one Ipir AO on each carbon atom. In the extended Hiickel treatment of nonplanar hydrocarbons, each valence MO contains contributions from four AOs on each carbon atom (one 2s and three 2p s) and one Is AO on each hydrogen atom. The AOs used are usually Slater-type orbitals with fixed orbital exponents determined from Slater s rules (Problem 15.79). For the simplified Hamiltonian (16.76), the problem separates into several one-electron problems ... 

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