A "Festschrift" volume fulfils a more far-reaching purpose than the laudatory one. It shows how science develops as a result of the activities - scientific and organizational - of an individual person. Scientific achievement cannot be subjected to the very refined measurement techniques of science itself, but there is a continuous mutual evaluation among scientists which manifests itself through refereeing, literature citation and dedicatory volumes like the present one. Near and distant associates of Per-Olov Lowdin were enthusiastic about the idea of a tribute to him in the form of a collection of scientific papers on the occasion of his sixtieth birthday. Monographs and journals have fairly well-defined readerships. This book is directed to a wider group of scientists. It presents reviews of areas where Lowdin's work has influenced the development as well as research papers with original results. We feel that it can serve as a source on the current status of the quantum theory of matter for scientists in neighbouring fields. It might also provide stimulus for renewed scientific efforts among scientists turned administrators and will certainly be relevant for teachers and students of quantum theory.

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.

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.

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.