Epilogue

Our intention in writing this book was to teach a theory that has been largely abandoned in the mainstream community of theoretical chemistry, but nevertheless is still being used every day by chemists in their thinking about molecules and reactions through the Lewis representation of molecules and Pauling's resonance theory ideas.Indeed, valance bond (VB) theory is a chemical theory, and its neglect undermines the cultural heritage of our science.This is all the more so since modern VB theory has evolved into an ab initio theory that has many levels, from the simple VBSCF (analogous of CASSCF) all the way to breathing-orbital VB (BOVB) and VBCI which include dynamic correlation (see Chapters 11 and 12).Therefore, for the sake of showing the versatility of VB theory, we have taken our reader along through 12 chapters, which describe different aspects of the theory, from its history and its rivalry with MO theory, through the development of a Hückel-type VB theory, all the way to the modern VBSCF, VBCI, and BOVB methods.The richness of VB theory is demonstrated through applications to chemical reactivity, photochemistry, and electronic structure of large molecules and on to the nature of excited-state species.Thus, this second edition of the book addresses quantitative aspects of the method, the existing computational methods and their domains, and the available program packages.In the end, we also provided some know-how on the nature of the input and output of VB calculations, and how to run meaningful calculations and obtain desirable quantities, such as resonance energies, bonding energies, excitation energies, and reaction barriers.Throughout this book, we have made an effort to create bridges between VB and MO theories and have shown the advantage of fusing the special insight of these two theories.Ultimately, researchers, students, and teachers using this edition of the book will determine the success of our effort.As we wrote in the preface: Any feedback will be most welcome!While the 12 chapters may seem plenty for a single course, the chemistry teacher has the freedom of selecting the subtopics.Thus, in the area of chemical bonding, VB theory predicts that alongside the traditional covalent and ionic electron-pair bonds, there exists a class where the covalent-ionic resonance energy is the root cause of the bonding, and sometimes of the existence, of the A Chemist's Guide to Valence Bond Theory: Insights into Chemical Bonding, Reactivity, and Excited States, Second Edition.Sason Shaik, David Danovich, and Philippe C. Hiberty.