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 perturbation 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.
