A guided tour through modern charge density analysis

Back in 1990, Philip Coppens - the author of the preface to this book and a reckoned authority in the field of X-ray charge-density (CD) studies, - wrote (with Dirk Feil) that "at present, charge density analysis is far from being a routine technique" [1]. However, less than 10 years later, in a paper significantly entitled Charge-Density Analysis at the Turn of the Century, he could be far more optimistic, and both about the easiness and the potential role of such an investigative tool [2]. According to Coppens, the CD analysis had eventually "developed into a technique now ready for application to a broad range of problems of interest in chemistry, biology and solid state physics" [2]. The present book is aimed at showing how such a prophecy has indeed been realized during the first decade of the new century, offering a balanced exposition of the most relevant recent applications of CD analysis, along with those theoretical, methodological and instrumental developments that have made them possible. Although this book is focused more on the experimental facets of the charge density analysis, a large emphasis is also given to those progresses of theoretical and computational quantum theory that allow useful and most necessary comparisons between theory and experiment [3], very often complementary. The book also covers, from many points of view and interest, aspects that go far beyond the "simple" charge density analysis, and highlight the relevance of joint charge, spin and momentumdensities studies, as well as of the role of one- and two-electron density matrices for discussing how atoms get bonded to one another in molecules and crystals. Even though CD has now come of age - in keeping with the title of a recent highlight published by Philip Coppens on Angewandte Chemie [4] - a comprehensive updated treatment of this broad field was definitely lacking. We hope that the over 20 chapters of this book might adequately satisfy this need. This chapter is simply aimed at introducing the reader to the whole book through a concise summary of basic aspects of CD analysis and a general overview of the CD research developments which took place over the last 10 years.While offering a coup d'oeil to those issues which are then developed in details in the remaining chapters, this first one also briefly highlights those few topics that despite their relevance could not find place in the book. This book is a comprehensive update of research on charge density analysis. On the other hand, a textbook like X-Ray Charge Densities and Chemical Bonding [5] by Philip Coppens and Atoms in Molecules, A Quantum Theory [6] by Richard Bader are more adequate for educational purposes and for a general introduction to this field. A recent book, edited by Cherif Matta and Russel Boyd [7] gives an update on advanced research, though limited to application of the Quantum Theory of Atoms in Molecules. A number of excellent reviews dealing with the diverse aspects of modern charge density analysis have also appeared during the last 10 years. They are not listed here, but properly referred to in the various chapters of this book.