This volume focuses on the use of quantum theory to understand and explain experiments in organic chemistry. High level ab initio calculations, when properly performed, are useful in making quantitative distinctions between various possible interpretations of structures, reactions and spectra. Chemical reasoning based on simpler quantum models is, however, essential to enumerating the likely possibilities. The simpler models also often suggest the type of wave function likely to be involved in ground and excited states at various points along reaction paths. This preliminary understanding is needed in order to select the appropriate higher level approach since most higher level models are designed to describe improvements to some reasonable zeroth order wave function. Consequently, most of the chapters in this volume begin with experimental facts and model functions and then progress to higher level theory only when quantitative results are required.In the first chapter, Zimmerman discusses a wide variety of thermal and photochemical reactions of organic molecules. Gronert discusses the use of ab initio calculations and experimental facts in deciphering the mechanism of β-elimination reactions in the gas phase. Bettinger et al focus on carbene structures and reactions with comparison of the triplet and singlet states. Next, Hrovat and Borden discuss more general molecules with competitive triplet and singlet contenders for the ground state structure. Cave explains the difficulties and considerations involved with many of the methods and illustrates the difficulties by comparing with the UV spectra of short polyenes. Jordan et al discuss long-range electron transfer using model compounds and model Hamiltonians. Finally, Hiberty discusses the breathing orbital valence bond model as a different approach to introducing the crucial σπ correlation that is known to be important in organic reactions. Contents:Some Theoretical Applications to Organic Chemistry (H E Zimmerman)Ab Initio Studies of Elimination Reaction Mechanisms (S Gronert)Computational Analyses of Prototype Carbene Structures and Reactions (H F Bettinger et al.)Violations of Hund's Rule in Organic Diradicals — Where to Look for Violations and How to Identify Them (D A Hrovat & W T Borden)Ab Initio Methods for the Description of Electronically Excited States: Survey of Methods and Selected Results (R J Cave)Long-Range Intramolecular Interactions: Implications for Electron Transfer (K D Jordan et al.)The Breathing Orbital Valence Bond Method (P C Hiberty)Readership: Graduate and postgraduate students in organic chemistry.Key Features:New materials from the last two decades, the crystallographic groups are presented from the very beginning with details proofs of the classical Bieberbach theorems