Extended Huckel method applications

The extended Huckel method [13] is an extension of the traditional Huckel method [14] expanding its range of applicability beyond planar conjugate systems. From a mathematical point of view, it consists simply in solving the matricial equation 1, where H is the hamiltonian matrix, C are... 

The extended Huckel method, as simple as it is, has proven enormously successful in very many applications. Its successes depend on the fact that the Fock matrices have the correct symmetry properties and that the atomic potentials supplied through the use of experimental ionization potentials are more or less correct. Extended Huckel theory (EHT) calculations are still being performed today and yield results of interest in nearly all areas of chemistry and condensed matter theory.As this theory requires only ionization potentials and basis sets, it has been extended in one form or another to all elements of the... 

Recent Applications of the Relatlvlstically Parameterized Extended Huckel Method on Actinide Compounds Pekka PYYKKO... 

Methods based on simulated Ab initio Molecular Orbital technique (SAMO) or cm the application of Linear Combination of Localized Orbitals have been proposed. The a priori advantages are a negligible cost (typically of the order of magnitude of an extended Huckel calculation) and the ab initio character of the approach. They suffer however from a rather tedious generation of a high number of matrix elements and it is still impossible to... 

By far, the theoretical approaches that experimental inorganic chemists are most familiar with and in fact nse to solve questions qnickly and qnalitatively are the simple Huckel method and Hoffinann s extended Hiickel theory. These approaches are nsed in concert with the application of symmetry principles in the bnUding of syimnetry adapted linear combinations (SALCs) or gronp orbitals. The ab initio and other SCF procednres ontlined above prodnce MOs that are treated by gronp theory as well, bnt that type of rigor is not usually necessary to achieve good qnahtative pictures of the character aud relative orderiugs of the molecular orbitals. 

In the preceding examples on the application of quantum-mechanical methods to the study of conformational problems in biochemistry, we have centered our attention essentially on results obtained by the PCILO method. The reason for this situation resides in the first place in the fact that the results obtained by this procedure are by far the most abundant. A second reason is, however, that they appear also the most satisfactory, being in particular superior to those obtained by the Extended Huckel Theory, which comes next after PCILO in the amount of work carried out. (For practical reasons, very few calculations have been performed in this field using the CNDO/2 method.)... 

Molecular-orbital theory has taken many forms and has been dealt with by many approximations. In 1963 Hoffmann S presented a formalism which he referred to as extended Hiickel (EH). In the 1930 s, however, this formalism would simply have been called molecular-orbital, since it is a straightforward application of molecular-orbital (MO) theory, using a one-electron Hamiltonian. Hoffmann referred to it as extended Hiickel because it did not limit itself to 7r-electron systems and was able to deal with saturated molecules by including all overlap integrals. In these respects it did extend the usual, or simple Huckel, method, which was customarily applied to 7T-electrons, and assumed complete tt — a separability. 

In support of this analysis, it was demonstrated that extended Huckel theory (EHT) correctly predicts a symmetrical transition state. Despite the substantial approximations of EHT, it does include overlap and thus closed-shell repulsion. In a clever "control experiment", a modified EHT code that does not include overlap produced an unsymmetrical transition state. It appears that the case of pericyclic transition states is one in which the approximations necessary to develop a rapid, semi-empirical computational model are too severe, and the semi-empirical methods are not applicable to such reactions. 

In recent years the fundamental ideas of Huckel molecular orbital theory, the Huckel rule, and other aspects of aromaticity have been extended to polyhedral three-dimensional inorganic structures regarded as aromatic like the two-dimensional aromatic hydrocarbons. Such an extension of Huckel molecular orbital theory requires recognition of its topological foundations so that they can be applied to three-dimensional structures as well as two-dimensional structures. In this connection graph theoretical methods can be used to demonstrate the close analogy between the delocalized bonding in two-dimensional planar aromatic systems such as benzene and that in three-dimensional deltahedral boranes, and carboranes. Related ideas can be shown to be applicable for metal carbonyl clusters, bare post-transition metal clusters, and polyoxometallates. ... 

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