This volume contains most of the invited lectures of the 2nd Structural Chemistry Indaba on "Molecular Interactions," held at Skukuza, Kruger Park, South Africa, August 3- 8, 1997. While the 1995 conference concentrated more on the principles underlying molecular modeling, like the existence of a molecular shape, this conference centers on molecular interactions or, more generally, on molecules in environments. Unfortunately, it was impossible, for various reasons, to unite all invited lectures in this volume, but nevertheless this collection contains contributions ranging from the fundamental quantum mechanical theory to recent research on organometallic crystals. For a summary, I would like to refer the reader to the introductory chapter by S.O. Sommerer, based on his concluding remarks at the conference. WemerGans for the editors v CONTENTS Intermolecular Interactions ...S. O. Sommerer Intermolecular Bonding ...3 1. C. A. Boeyens Chemical Reactions in the Framework of Single Quantum Systems ...9 A. Amann The Molecule and Its Environment ...25 ...B. T. Sutcliffe Dynamic Aspects of Intermolecular Interactions ...49 ...1. F. Ogilvie Atomic Interactions and the Charge Density ...5 7 ...T. Koritsanszky 71 Cyclometallation of Alkylphosphines M. T. Benson and T. R. Cundari C-H-. *0 Hydrogen Bonds in Organometallic Crystals 83 D. Braga and F. Grepioni The Importance of Intra-and Intermolecular Weak Bonds in Transition Metal Coordination Compounds ...97 P. Comba Relationships between Experiment and Theory in the Study of Intermolecular Interactions ...111 ...

At the end of the workshop on "New Theoretical Concepts in Physical Chemistry," one of the participants made an attempt to present a first impression of its achievements from his own personal standpoint. Appar ently his views reflected a general feeling, so that the organizers thought they would be suitable as a presentation of the proceedings for future readers. That is the background from which this foreword was born. The scope of the workshop is a very broad one. There are contribu tions from mathematics, physics, crystallography, chemistry and biology; the problems are approached either by means of axiomatic and rigorous methods, or at an empirical phenomenological level. This same diversifi cation can be found in the new basic concepts presented. Some arise from pure theoretical investigation in C*-algebra or in quantum probability theory; others from an analysis of very complex experimental data like nuclear energy levels, or processes on the frontier between classical and quantum physics; others again have their origin in the discovery of new ordered structures like the icosahedral crystal phases, or the knots of DNA molecules; others follow from the application of ideas like frac tals or chaos to new fields like spectral theory or chemical reactions. It is to be expected that readers will have to face the same sort of difficulties as did the participants in understanding such diverse languages, in applying themselves to subjects possibly far from their own experience, and in grasping highly sophisticated new concepts."

This NATO Advanced Study Institute centered on large-scale molecular systems: Quantum mechanics, although providing a general framework for the description of matter, is not easily applicable to many concrete systems of interest; classical statistical methods, on the other hand, allow only a partial picture of the behaviour of large systems. The aim of the ASI was to present both aspects of the subject matter and to foster interaction between the scientists working in these important areas of theoretical physics and theoretical chemistry. The quantum-mechanical part was mostly based on the operator-algebraic formulation of quantum mechanics and comprised quantum statistics of infinite systems with special em phasis on macroscopic observables, equilibrium conditions, irreversibility on the one hand, symmetry breaking for molecules in the radiation field and macroscopic quantum phenomena in the theory of superconductivity (BCS-theory) on the other hand. In addition, phase-space methods for many-body systems were also presented. Statistical physics was the main topic in the other lectures of the School; much emphasis was put on the statistical features of macros copic ("large") systems, the lectures dealt with mass and energy transport im polymers, in gels and in microemulsions, with aggregation and growth phenomena, with relaxation in complex, correlated systems, with conduction and optical properties of polymers, and with the means of describing disordered systems, above all fractals and related hierarchical models.

This volume contains most of the invited lectures of the 2nd Structural Chemistry Indaba on "Molecular Interactions," held at Skukuza, Kruger Park, South Africa, August 3- 8, 1997. While the 1995 conference concentrated more on the principles underlying molecular modeling, like the existence of a molecular shape, this conference centers on molecular interactions or, more generally, on molecules in environments. Unfortunately, it was impossible, for various reasons, to unite all invited lectures in this volume, but nevertheless this collection contains contributions ranging from the fundamental quantum mechanical theory to recent research on organometallic crystals. For a summary, I would like to refer the reader to the introductory chapter by S.O. Sommerer, based on his concluding remarks at the conference. WemerGans for the editors v CONTENTS Intermolecular Interactions ...S. O. Sommerer Intermolecular Bonding ...3 1. C. A. Boeyens Chemical Reactions in the Framework of Single Quantum Systems ...9 A. Amann The Molecule and Its Environment ...25 ...B. T. Sutcliffe Dynamic Aspects of Intermolecular Interactions ...49 ...1. F. Ogilvie Atomic Interactions and the Charge Density ...5 7 ...T. Koritsanszky 71 Cyclometallation of Alkylphosphines M. T. Benson and T. R. Cundari C-H-. *0 Hydrogen Bonds in Organometallic Crystals 83 D. Braga and F. Grepioni The Importance of Intra-and Intermolecular Weak Bonds in Transition Metal Coordination Compounds ...97 P. Comba Relationships between Experiment and Theory in the Study of Intermolecular Interactions ...111 ...