Valence bond - Rebirth of the phoenix or relic from the stone age

The valence bond (VB) method has enjoyed its prime time during the early stages in the field of quantum chemistry. After the advent of molecular orbital methods VB lost its popularity but continued to be improved and refined by a small community of scientists who appreciated its power of revealing insight into the origins of chemical reactivity. This review summarizes the developments of the VB theory in the past few decades by focusing on two major areas of research: studies of the reactivity of small chemical systems and discovering the origins of enzyme catalysis. In both cases the unique capabilities of VB that facilitated discoveries of new concepts in an elegant and seemingly effortless way are discussed. It is suggested that owing to the success of these discoveries VB methodology is once again steadily gaining momentum. It is believed that VB concepts will play a major role in the future of theoretical chemistry.