Quantum chemistry natural bond orbital theory

The recent developments in generalized Valence Bond (GVB) theory have been reviewed by Goddard and co-workers,13 and also the use of natural orbitals in theoretical chemistry,14 15 and the accuracy of computed one-electron properties.18 The Xa method has been reviewed by Johnson,17 and Hurley has discussed high-accuracy calculations on small molecules.18 Several other reviews of interest have appeared in Advances in Quantum Chemistry.17 Localized orbital theory has been reviewed by England, Salmon, and Ruedenberg,19 and the bonding in transition-metal complexes discussed by Brown et a/.20 Finally, the recent developments in computational quantum chemistry have been reviewed by Hall.21... 

Practitioners of quantum chemistry employed both the visual imagery of nineteenth-century theoretical chemists like Kekule and Crum Brown and the abstract symbolism of twentieth-century mathematical physicists like Dirac and Schrodinger. Pauling s Nature of the Chemical Bond abounded in pictures of hexagons, tetrahedrons, spheres, and dumbbells. Mulliken s 1948 memoir on the theory of molecular orbitals included a list of 120 entries for symbols and words having exact definitions and usages in the new mathematical language of quantum chemistry. 

The contributions of Erich Hiickel to the development of molecular orbital theory have already been mentioned in the subsection on Germany (Section 5.4.1) the development of semi-empirical quantum mechanical treatments in organic chemistry by M. J. S. Dewar has been discussed in Section 5.5. In the early development of the application of quantum mechanics to chemistry, Linus Pauling (1901-1994)359 was pre-eminent. He was associated with CalTech for most of his career. His work before World War II generated two influential books the Introduction to Quantum Mechanics (with E. Bright Wilson, 1935)360 and The Nature of the Chemical Bond (1939).361 He favoured the valence-bond treatment and the theory of resonance. 

Valence-Bond (VB) and Molecular-Orbital (MO) theories both were clearly formulated by the end of the first decade of quantum mechanics. Of course VB theory is connected to early conceptual roots in chemistry, as emphasized by Rumer [1] and more particularly by Pauling, in a review [2] and then in his masterwork [3] The Nature cfiJte Chemical Bond. Thence for some jjeriod of time VB theory seems in the chemical community to have been viewed quite favorably. 

Slater was one of the first scientists to realize that in qualitative discussions of molecule formation, the best procedure to follow was to compare critically the results of the two different methods developed—the valence bond and the molecular orbital viewpoint, and to point out, as already mentioned, that the choice between the two should be made on the basis of convenience rather than correctness. However, in Introduction to Chemical Physics, just a few lines are devoted to such a central topic in the context of Slater s contributions to quantum chemistry as well as in the development of the discipline itself. In the chapter on "Interatomic and Intermolecular Forces," in the section about "Exchange Interactions Between Atoms and Molecules," Slater stated that the problems of quantum chemistry are among the most complicated of quantum theory and that the theory itself will not be treated in an analytical manner. He considered the Heider-London approach and the molecular orbital approach as two different approximate methods of calculation used in wave mechanics. He believed that these two methods do not differ in their "fundamentals, but in the precise nature of the analytical steps used." And he proposed to study "the fundamental physical processes behind the intermolecular actions and we shall find that... 

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