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This book is devoted to a general discussion about localization and delocalization in quantum chemistry. The first volume is concerned with molecules in their ground state. It is made of papers presented during the academic year 73-74 at an international seminar organized by some members of the 'Centre de Mecanique Ondulatoire Appli- quee du C.N.R.S.' and some members of the 'Laboratoire de Chimie Quantique de l'Institut de Biologie Physico-Chimique'. It contains also reports of discussions which followed the presentation of invited papers. It is a 'forum' in which each expert gives his opinion on a work in progress. The volume is divided into four parts. The first one is a statistical analysis of the localizability of molecular electrons in the three-dimensional space. It contains an exposition of the basic ideas of the loge theory which provides a framework to do such an analysis. The second part is concerned with the separability of a molecular wave function and its expression in terms of localized elements. An exploration is made of the rela- tionship between the localizability of electrons and the possibility of expressing the wave function in terms of localized orbitals.The third part is devoted to the partition of the energy in local contributions.

The second volume of the proceedings of the international seminar devoted to locali· zation and delocalization in quantum chemistry is divided into four parts. The first one is mainly concerned with the localizability of electrons in ionized and exited states. The second part shows how is it possible to take advantage of the localizability of electrons to compute molecular wave-functions. The third part of the book is an homogeneous analysis of the electronic collective excitation and of the motion of excitons in organic solids. The last section is devoted to the study of the role of electron localizability in the chemical reactivity of molecules. Concluding remarks are concerned with a careful analysis of the localizability concept itself in relation with a possible interpretation of the wave-mechanics. PART I ELECTRON LOCALIZATION IN IONIZED AND EXCITED STATES APPLICATIONS OF PAIR DENSITY ANALYSIS R. CONSTANCIEL and L. ESNAULT Centre de Mecanique Ondulatoire Appliquee, Paris, France Abstract. The method of pair density analysis is applied to various kinds of calculations. We examine the influence of the quality of the wavefunction and of the nuclear configuration; the problem of hybridization is discussed, as weIl as the relations between separability and excitation.

The second volume of the proceedings of the international seminar devoted to locali· zation and delocalization in quantum chemistry is divided into four parts. The first one is mainly concerned with the localizability of electrons in ionized and exited states. The second part shows how is it possible to take advantage of the localizability of electrons to compute molecular wave-functions. The third part of the book is an homogeneous analysis of the electronic collective excitation and of the motion of excitons in organic solids. The last section is devoted to the study of the role of electron localizability in the chemical reactivity of molecules. Concluding remarks are concerned with a careful analysis of the localizability concept itself in relation with a possible interpretation of the wave-mechanics. PART I ELECTRON LOCALIZATION IN IONIZED AND EXCITED STATES APPLICATIONS OF PAIR DENSITY ANALYSIS R. CONSTANCIEL and L. ESNAULT Centre de Mecanique Ondulatoire Appliquee, Paris, France Abstract. The method of pair density analysis is applied to various kinds of calculations. We examine the influence of the quality of the wavefunction and of the nuclear configuration; the problem of hybridization is discussed, as weIl as the relations between separability and excitation.