"Preface In the second half of the nineteenth century Physical chemistry developed as a well defined subject, consisting of thermodynamics, kinetics, and transport processes, and mainly dealing with bulk properties and continuum models. When quantum mechanics was discovered in 1925, paving the way for modern molecular physics, this subject was less well received by the chemists. Partly this was due to the morass of equations and calculations one sinks into, just to get insight into such a simple concept as the chemical bond. Application of quantum methods in chemistry was pioneered by people like Henry Eyring, Linus Pauling, Robert Mulliken, Per-Olov Löwdin, Björn Roos, and many others. Most of these scientists called themselves "quantum chemists". The calculation of electronic energies and wave functions made impressive progress during the second half of the twentieth century, a development that ran parallel to the improvement of computer technology. At the same time the new subject of Quantum Chemistry became standardized and automatized. There is now an army of users of ready programs, who calculate bond lengths and reaction barriers with ever increasing accuracy. Unfortunately there appears to be some hesitation to tackle more difficult problems, such as transfer of protons, electrons, and excitations. These fields were pioneered by, for example, Rudy Marcus, Norman Sutin, Noel Hush, Joshua Jortner, John R. Miller, and Ahmed Zewail. It has become customary to refer to physics and chemistry based on the quantum mechanical behavior of the elementary particles, atoms and molecules, as Chemical Physics, and this explains the main title of this book"