This volume is the first of a set of two which contain the invited lectures given at the international seminar of the same title held at the Centre de Mecanique Ondulatoire Appliquee du Centre National de la Recherche Scientifique in Paris (France) from October 1983 to May 1985. They are intended to provide a survey of topics of current interest relative to the structure and the dynamics of molecular systems. The papers have been selected on the basis of their relevance to the following four topics: i) molecular conformations and transformations; ii) molecular relaxation and motion; iii) charge, spin and momentum distributions in molecular solids; iv) collective phenomena in condensed matter. The first volume deals f)1ostly with the first two topics, the second volume mostly with the last two. Each volume consists of about fifteen self contained, reference contributions covering recent achievements in active branches of molecular physics and physical chemistry. The first four papers of the present volume deal with theoretical aspects of structure and reactivity problems, with particular attention being paid to topology considerations, which have joined symmetry con siderations as an important tool in approaching chemistry problems. The treatment of nuclear probability density distributions is performed on a model basis for a simple system, even though it has come to the attention of theoreticians through experimental results for complex systems.

This volume is the second of a set of two which contain 28 selected from the l. j. O invited lectures given at the internatim;al seminar of the same title held at the Centre de Mecanique Ondulatoire Appliquee du Centre National de la Recherche Scientifique in Paris (France) from October 1983 to May 1985. They are intended to provide a survey of topics of current interest relative to the structure and the dynamics of molecular systems. The papers have been selected on the basis of their relevance to the following four topics: i) molecular conformations and transformations; H) molecular relaxation and motion; iii) charge, spin and momentum distributions and intermolecular interactions; iv) collective phenomena in condensed matter. The first volume deals mostly with the first two topics, the second volume mostly with the last two. The two volumes consist of an approximately equal number of self-con tained, reference contributions covering recent achievements in active branches of molecular physics and physical chemistry. The first two papers of the present volume deal with theoretical aspects of intermolecular interactions: the first paper with the physical origin of the so-called non-exchange molecular terms, a complete deriva tion of which is given using Rayleigh-Schrodinger second-order perturba tion theory; the second paper with the symmetry analysis of the effects of interactions between rigid molecules and crystal environments, using the isodynamic-group theoretical approach devised by Altmann for non rigid systems.

This volume contains a selection of scientific papers related to the structure and dynamics of non-rigid molecules. This frontline topic was born a few decades ago, when Longuet-Higgins proposed his famous theory of Molecular Symmetry Groups (Mol. Phys. 6, (1962) 457). Unfortunately, since this early paper, very few publications have been devoted to the study of non-rigid molecules. Let us mention some books which dedicate some chapters to them: Induced Representations in Crystals and Molecules, by S. L. Altmann, Academic Publishers, 1977; Molecular Symmetry and Spectroscopy, by P. R. Bunker, Academic Publishers, 1979; and finally Large Amplitude Motion in Molecules, Vols. I and II, by several authors, Springer Verlag, 1979. More recently an International Symposium on Non-Rigid Molecules was held in Paris, France, from 1-7 July 1982, the proceedings of which were published in the volume entitled Symmetries and Properties of Non-Rigid Molecules. A Comprehensive Survey, edited by J. Maruani et al., Elsevier, 1983. Finally, we should mention the very specialized work The Permutational Approach to Dynamic Stereochemistry, by J. Brocas et al., McGraw-Hill, 1983. The purpose of this book is to fill in this information on the structure and dynamics of non-rigid systems. To this aim, we have gathered a collection of recent papers written by the most qualified specialists in the world, covering a large field from van der Waals molecules to inorganic complexes and organic polyrotor molecules, as well as considering statistical and dynamic aspects.

This volume is the second of a set of two which contain 28 selected from the l. j. O invited lectures given at the internatim;al seminar of the same title held at the Centre de Mecanique Ondulatoire Appliquee du Centre National de la Recherche Scientifique in Paris (France) from October 1983 to May 1985. They are intended to provide a survey of topics of current interest relative to the structure and the dynamics of molecular systems. The papers have been selected on the basis of their relevance to the following four topics: i) molecular conformations and transformations; H) molecular relaxation and motion; iii) charge, spin and momentum distributions and intermolecular interactions; iv) collective phenomena in condensed matter. The first volume deals mostly with the first two topics, the second volume mostly with the last two. The two volumes consist of an approximately equal number of self-con tained, reference contributions covering recent achievements in active branches of molecular physics and physical chemistry. The first two papers of the present volume deal with theoretical aspects of intermolecular interactions: the first paper with the physical origin of the so-called non-exchange molecular terms, a complete deriva tion of which is given using Rayleigh-Schrodinger second-order perturba tion theory; the second paper with the symmetry analysis of the effects of interactions between rigid molecules and crystal environments, using the isodynamic-group theoretical approach devised by Altmann for non rigid systems.