Modern Charge Density Analysis

This is the preface to the book by Philip Coppens : The contributions in this book manifest the tremendous developments that have taken place in the last 10-20 years in electron density research in the broadest sense. Although the possibility of measuring the distribution of electrons in crystalline solids with X-rays was realized soon after the discovery of X-ray diffraction early in the twentieth century it was not until half a century later that the first systematic attempts started to bear fruit. This was of course well after the advent of quantum mechanics but before the development of fast computers made the calculation of electronic structure a tool employed by a large research community in the physical sciences. Already in 1922 Bragg realized that measurement of the net charges in NaCl was as yet impossible, but not beyond reach, when he wrote (with James and Bosanquest) that 'it seems that crystal analysis must be pushed to a far Greater degree of refinement before it can settle the point' (Phil. Mag44, 433 (1922). The 'greater degree of refinement' has now been achieved to a degree well beyond what could be envisioned in 1922. This applies to both experiment and theory and to the broad field of charge spin and momentum densities, all three of which are discussed in the current volume. The development of radiation sources, detectors and automation of data collection equipment has been nothing short of dramatic. New theoretical methods and in particular Bader's development of the densitybased Theory of Atoms in Molecules, combined with the exponential increase in computing power have created a link between theory and experiment which opened a new phase in electron density research in which the emphasis is very much on interpretation of the results and their application in the understanding of chemical, physical and biological phenomena. The current volume is highly timely in covering the advances. Its chapters survey the state of the art and point the way to further exciting developments.