This book presents new and important research from around the world in quantum chemistry which is a branch of theoretical chemistry. Quantum chemistry applies quantum mechanics and quantum field theory to address issues and problems in chemistry. The description of the electronic behaviour of atoms and molecules as pertaining to their reactivity is one of the applications of quantum chemistry. Quantum chemistry lies on the border between chemistry and physics, and significant contributions have been made by scientists from both fields. It has a strong and active overlap with the field of atomic physics and molecular physics, as well as physical chemistry.