Ab Initio Valence Calculations in Chemistry describes the theory and practice of ab initio valence calculations in chemistry and applies the ideas to a specific example, linear BeH2. Topics covered include the Schrödinger equation and the orbital approximation to atomic orbitals; molecular orbital and valence bond methods; practical molecular wave functions; and molecular integrals. Open shell systems, molecular symmetry, and localized descriptions of electronic structure are also discussed. This book is comprised of 13 chapters and begins by introducing the reader to the use of the Schrödinger equation to solve the electronic structure of molecular systems. This discussion is followed by two chapters that describe the chemical and mathematical nature of orbital theories in quantum chemistry. Two general ways of using chemical and physical information in looking for approximate solutions of the Schrödinger equation are highlighted: model approximations and numerical approximations. Attention then turns to atomic orbitals as the basis of a description of molecular electronic structure; practical molecular wave functions; and a general strategy for performing molecular valence calculations. The final chapter examines the nature of the valence electronic structure by using invariance with respect to transformations among the occupied molecular orbitals and among the atomic orbitals. This text will be of interest to students and practitioners of chemistry, biochemistry, and quantum mechanics.

Until recently quantum chemical ab initio calculations were re stricted to atoms and very small molecules. As late as in 1960 Allen l and Karo stated : "Almost all of our ab initio experience derives from diatomic LCAO calculations ••• N and we have found in the litera ture "approximately eighty calculations, three-fourths of which are for diatomic molecules ••• There are approximately twenty ab initio calculations for molecules with more than two atoms, but there is a decided dividing line between the existing diatomic and polyatomic wave functions. Confidence in the satisfactory evaluation of the many -center two-electron integrals is very much less than for the diatom ic case". Among the noted twenty calculations, SiH was the largest 4 molecule treated. In most cases a minimal basis set was used and the many-center two-electron integrals were calculated in an approximate way. Under these circumstances the ab initio calculations could hard ly provide useful chemical information. It is therefore no wonder that the dominating role in the field of chemical applications was played by semiempirical and empirical methods. The situation changed essentially in the next decade. The problem of many-center integrals was solved, efficient and sophisticated computer programs were devel oped, basis sets suitable for a given type of problem were suggested, and, meanwhile, a considerable amount of results has been accumulated which serve as a valuable comparative material. The progress was of course inseparable from the development and availability of computers.

Publisher Description

This book is intended as a guide to the ab initio calculation of molecular structure and properties. It provides the necessary working information to enable the non-specialist to use and understand electronic structure methods and related computing technology, despite the high level of sophistication of quantum chemical methods. The initial chapters define and outline theoretical concepts, methods and computational approaches. Descriptive information and definitions of the terminology are given first; more detailed and mathematical explanations follow. These first chapters thus provide the background information needed to use the extensive literature of ab initio electronic structure theory. The next chapter first provides an overview of the technical issues relating to molecular properties, and then gives a rather detailed but general development. The latter part of this chapter is mainly intended for those first encountering the methodologies of properties determination and intending to pursue further developments. The other chapters provide reviews of calculations in the literature and assessments of factors influencing accuracy. The book is particularly useful to those who need a working understanding of ab initio calculations and well-suited to graduate students and researchers in computational and theoretical chemistry, researchers in electronic structure, spectroscopists and organic chemists.

Diatomic Molecules: Results of Ab Initio Calculations provides the results obtained from quantum-mechanical calculations on the electronic structure of diatomic molecules. This six-chapter text also discusses the related concepts of ab initio calculation methods. This book considers first the primary methods used in the computation of molecular wave functions and of related properties. This topic is followed by discussions on the linear combination of atomic orbital and linear combination of mixed atomic orbital approximations and basis sets; electronic population analysis; spectroscopic transition probabilities; and the nature of chemical bonding. The remaining chapters examine the features of various theories that become prominent when two or more electrons are present, or are important in hydrides or homopolar and heteropolar molecules. This text will be of great value to organic and inorganic chemists and physicists.

Valence bond (VB) theory, which builds the descriptions of molecules from those of its constituent parts, provided the first successful quantum mechanical treatments of chemical bonding. Its language and concepts permeate much of chemistry, at all levels. Various modern formulations of VB theory represent serious tools for quantum chemical studies of molecular electronic structure and reactivity. In physics, there is much VB-based work (particularly in semi-empirical form) on larger systems. Importance of TopicThe last decade has seen significant advances in methodology and a vast increase in the range of applications, with many new researchers entering the field.Why This TitleValence Bond Theory succeeds in presenting a comprehensive selection of contributions from leading valence bond (VB) theory researchers throughout the world. It focuses on the vast increase in the range of applications of methodology based on VB theory during the last decade and especially emphasizes recent advances.