The quantitative success that quantum chemistry has had since the early-1970s has led to the launch of "Conceptual Trends in Quantum Chemistry", a collection of essays intended to stimulate discussion in this field, of which the first volume was published in 1994, and the second in 1995. This third volume contains 14 papers covering topics such as recent developments in multiple scattering theory and density functional theory for molecules and solids; localized atomic hybrids; quantum electrodynamics and molecular structure; aspects of the chemical bond; Lie symmetries in quantum mechanics; the interplay between quantum chemistry and molecular dynamics simulations; the permutation group in many-electron theory; new developments in many body perturbation theory and coupled cluster theory; a philosopher's perspective of the "problem" of molecular shape; Van Der Waals interactions from density functional theories; different legacies and common aims; potential energy hypersurfaces for hydrogen bonded clusters (HF)N; one-electron pictures of electronic structure; and shape in quantum chemistry. This text should be of interest to researchers and graduate students whose work involves quantum chemistry, quantum mechanics, chemical physics, methodology, and mathematical statistical methods.

A companion volume to "Conceptual Trends in Quantum Chemistry", this work contains eight contributions focusing on important conceptual trends in atomic and molecular theory. The "polarization" between ab initio and semi-empirical methods is thoroughly analyzed in two of the articles, which also provide bridges between such procedures. Hydrogen-transfer theory and electron delocalization are treated in two further papers. Explicitly time-dependent descriptions of intermolecular dynamics, which constitute a characteristic trend in current research, are represented by an article about the quantum dynamics of diatoms in external fields. A view of certain atomic excited states is presented in a paper on collective and independent particle character, and a new theoretical tool is surveyed in an article on dimensional scaling. The final article analyzes density functional theory.

I would like to present to a wide circle of the readers working in quantum chem- istry and solid-state physics, as ,,*ell as in other fields of many-body physics and its interfaces, this book deyoted to density functional theory written by my colleagues Eugene S. Kryachko and Eduardo Y. Ludena. Their ways to this theory are rather different although basically both of them are quantum chemical. Eugene S. Kryachko came to energy density functional theory from the theory of reduced density matrices, and Eduardo \'. Ludena dewloped earlier the concept of loges in quantum chemistry. Neyertheless, their earlier interests giw the possibility to consolidate and formulate energy density functional theory in a unified and consistent way, in my opinion. Raymond Daudel Paris ACKNOWLEDGMENTS The authors are indebted to Carl Almbladh, Victor Va. Antonchenko, John Avery, Richard F. W. Bader, Ulf \'on Barth, Jean-Louis Calais, A. John Coleman, Jens P. Dahl, Robert Donnelly, Harold Englisch, Robert 1\1. Erdahl, Oswaldo Goscinski, John E. Harriman, Gintas Kamuntavichius, Illja G. Kaplan, Jaime Keller, \'alentin Khart- siev, Toshikatsu Koga, Per-Olov Lo\ydin, T. Tung Nguyen-Dang, Ivan Zh. Petkov, Jerome K. Percus, l\lary Beth Ruskai, John R. Sabin, Zdenek Slanina, \'ladimir Shi- rokov, l\lario V. Stoitsov, Yoram Tal, and \Vaitao Yang, who in one way or another, either through their kind support, help, discussions or valuable comments created the human and intellectual background which made this book possible.

This volume contains nine contributions, from leading scientists, which embrace the fundamentals of various aspects of the conceptual development of quantum chemistry. Topics dealt with include the behaviour of molecules in magnetic fields, the long-standing problem of the decoupling of nuclear from electron motion in molecules, the status of density functional theory, and the string model of chemical reactions. Insights into basic concepts are also presented, such as the nature of chemical bonding and molecular structure and the quantum mechanical problem of the phase space. Trends in the mathematical base of quantum chemistry, such as the methods of hyperspherical harmonics and of the wavelet transform are discussed. This work should be useful for researchers and graduate students of quantum and theoretical chemistry, quantum mechanics and chemical physics.

Per-Olov Lowdin's stature has been a symbol of the world of quantum theory during the past five decades, through his basic contributions to the development of the conceptual framework of Quantum Chemistry and introduction of the fundamental concepts; through a staggering number of regular summer schools, winter institutes, innumerable lectures at Uppsala, Gainesville and elsewhere, and Sanibel Symposia; by founding the International Journal of Quantum Chemistry and Advances in Quantum Chemistry; and through his vision of the possible and his optimism for the future, which has inspired generations of physicists, chemists, mathematicians, and biologists to devote their lives to molecular electronic theory and dynamics, solid state, and quantum biology.

Fundamental World of Quantum Chemistry Volumes I, II and III form a collection of papers dedicated to the memory of Per-Olov Lowdin. These volumes are of interest to a broad audience of quantum, theoretical, physical, biological, and computational chemists; atomic, molecular, and condensed matter physicists; biophysicists; mathematicians working in many-body theory; and historians and philosophers of natural science. The volumes will be accessible to all levels, from students, PhD students, and postdocs to their supervisors."

Per-Olov Lowdin's stature is a symbol of the world of quantum theory during the past five decades, through his basic contributions to the development of the conceptual framework of Quantum Chemistry and introduction of the fundamental concepts; through a staggering number of regular summer schools, winter institutes, innumerable lectures at Uppsala, Gainesville and elsewhere, and Sanibel Symposia; by founding the International Journal of Quantum Chemistry and Advances of Quantum Chemistry, and through his vision of the possible and his optimism for the future, which has inspired generations of physicists, chemists, mathematicians, and biologists to devote their lives to molecular electronic theory and dynamics, solid state, and quantum biology.
Fundamental World of Quantum Chemistry forms a collection of papers dedicated to the memory of Per-Olov Lowdin. These volumes are of interest to a broad audience of quantum, theoretical, physical, biological, and computational chemists; atomic, molecular, and condensed matter physicists; biophysicists; mathematicians working in many-body theory; and historians and philosophers of natural science. The volumes will be accessible to all levels, from students, PhD students, and postdocs to their supervisors."

There can be no doubt about the tremendous quantitative success that quantum chemistry has had in the last three decades. This has led to the launch of Conceptual Trends in Quantum Chemistry, a collection of essays intended to stimulate discussion in this field, of which the first volume was published in 1994, and the second in 1995. This third volume contains fourteen papers by leading experts, covering topics such as recent developments in multiple scattering theory and density functional theory for molecules and solids; localised atomic hybrids; quantum electrodynamics and molecular structure; aspects of the chemical bond; Lie symmetries in quantum mechanics; the interplay between quantum chemistry and molecular dynamics simulations; the permutation group in many-electron theory; new developments in many body perturbation theory and coupled cluster theory; a philosopher's perspective of the problem' of molecular shape; Van Der Waals interactions from density functional theories; different legacies and common aims; potential energy hypersurfaces for hydrogen bonded clusters (HF)N; one-electron pictures of electronic structure; and shape in quantum chemistry. Audience: This book will be of interest to researchers and graduate students whose work involves quantum chemistry, quantum mechanics, chemical physics, methodology, and mathematical statistical methods.

The rivers run into the sea, yet the sea is not full Ecclesiastes What is quantum chemistry? The straightforward answer is that it is what quan tum chemists do. But it must be admitted, that in contrast to physicists and chemists, "quantum chemists" seem to be a rather ill-defined category of scientists. Quantum chemists are more or less physicists (basically theoreticians), more or less chemists, and by and large, computationists. But first and foremost, we, quantum chemists, are conscious beings. We may safely guess that quantum chemistry was one of the first areas in the natural sciences to lie on the boundaries of many disciplines. We may certainly claim that quantum chemists were the first to use computers for really large scale calculations. The scope of the problems which quantum chemistry wishes to answer and which, by its unique nature, only quantum chemistry can answer is growing daily. Retrospectively we may guess that many of those problems meet a daily need, or are say, technical in some sense. The rest are fundamental or conceptual. The daily life of most quantum chemists is usually filled with grasping the more or less technical problems. But it is at least as important to devote some time to the other kind of problems whose solution will open up new perspectives for both quantum chemistry itself and for the natural sciences in general.

The rivers run into the sea, yet the sea is not full Ecclesiastes What is quantum chemistry? The straightforward answer is that it is what quan tum chemists do. But it must be admitted, that in contrast to physicists and chemists, "quantum chemists" seem to be a rather ill-defined category of scientists. Quantum chemists are more or less physicists (basically theoreticians), more or less chemists, and by large, computationists. But first and foremost, we, quantum chemists, are conscious beings. We may safely guess that quantum chemistry was one of the first areas in the natural sciences to lie on the boundaries of many disciplines. We may certainly claim that quantum chemists were the first to use computers for really large scale calculations. The scope of the problems which quantum chemistry wishes to answer and which, by its unique nature, only quantum chemistry can only answer is growing daily. Retrospectively we may guess that many of those problems meet a daily need, or are say, technical in some sense. The rest are fundamental or conceptual. The daily life of most quantum chemists is usually filled with grasping the more or less technical problems. But it is at least as important to devote some time to the other kind of problems whose solution will open up new perspectives for both quantum chemistry itself and for the natural sciences in general.

I would like to present to a wide circle of the readers working in quantum chem- istry and solid-state physics, as ,,*ell as in other fields of many-body physics and its interfaces, this book deyoted to density functional theory written by my colleagues Eugene S. Kryachko and Eduardo Y. Ludena. Their ways to this theory are rather different although basically both of them are quantum chemical. Eugene S. Kryachko came to energy density functional theory from the theory of reduced density matrices, and Eduardo \'. Ludena dewloped earlier the concept of loges in quantum chemistry. Neyertheless, their earlier interests giw the possibility to consolidate and formulate energy density functional theory in a unified and consistent way, in my opinion. Raymond Daudel Paris ACKNOWLEDGMENTS The authors are indebted to Carl Almbladh, Victor Va. Antonchenko, John Avery, Richard F. W. Bader, Ulf \'on Barth, Jean-Louis Calais, A. John Coleman, Jens P. Dahl, Robert Donnelly, Harold Englisch, Robert 1\1. Erdahl, Oswaldo Goscinski, John E. Harriman, Gintas Kamuntavichius, Illja G. Kaplan, Jaime Keller, \'alentin Khart- siev, Toshikatsu Koga, Per-Olov Lo\ydin, T. Tung Nguyen-Dang, Ivan Zh. Petkov, Jerome K. Percus, l\lary Beth Ruskai, John R. Sabin, Zdenek Slanina, \'ladimir Shi- rokov, l\lario V. Stoitsov, Yoram Tal, and \Vaitao Yang, who in one way or another, either through their kind support, help, discussions or valuable comments created the human and intellectual background which made this book possible.

Per-Olov Lowdin's stature has been a symbol of the world of quantum theory during the past five decades, through his basic contributions to the development of the conceptual framework of Quantum Chemistry and introduction of the fundamental concepts; through a staggering number of regular summer schools, winter institutes, innumerable lectures at Uppsala, Gainesville and elsewhere, and Sanibel Symposia; by founding the International Journal of Quantum Chemistry and Advances in Quantum Chemistry; and through his vision of the possible and his optimism for the future, which has inspired generations of physicists, chemists, mathematicians, and biologists to devote their lives to molecular electronic theory and dynamics, solid state, and quantum biology.

Fundamental World of Quantum Chemistry Volumes I, II and III form a collection of papers dedicated to the memory of Per-Olov Lowdin. These volumes are of interest to a broad audience of quantum, theoretical, physical, biological, and computational chemists; atomic, molecular, and condensed matter physicists; biophysicists; mathematicians working in many-body theory; and historians and philosophers of natural science. The volumes will be accessible to all levels, from students, PhD students, and postdocs to their supervisors."

Per-Olov LAwdin's stature is a symbol of the world of quantum theory during the past five decades, through his basic contributions to the development of the conceptual framework of Quantum Chemistry and introduction of the fundamental concepts; through a staggering number of regular summer schools, winter institutes, innumerable lectures at Uppsala, Gainesville and elsewhere, and Sanibel Symposia; by founding the International Journal of Quantum Chemistry and Advances of Quantum Chemistry, and through his vision of the possible and his optimism for the future, which has inspired generations of physicists, chemists, mathematicians, and biologists to devote their lives to molecular electronic theory and dynamics, solid state, and quantum biology.
Fundamental World of Quantum Chemistry forms a collection of papers dedicated to the memory of Per-Olov LAwdin. These volumes are of interest to a broad audience of quantum, theoretical, physical, biological, and computational chemists; atomic, molecular, and condensed matter physicists; biophysicists; mathematicians working in many-body theory; and historians and philosophers of natural science. The volumes will be accessible to all levels, from students, PhD students, and postdocs to their supervisors.