This book deals with quantum chemistry providing comprehensive information regarding theoretical basis and novel applications in the field. Until early 20th century, many questions regarding molecules like their existence in stable and rigid networks between atoms and changing into different types of molecules had been unanswered. With the discovery of quantum mechanics, this scenario changed. Such queries were especially important, given the fact that they form the basis of life. Quantum mechanics is the theory for small particles such as electrons and nuclei. Heitler and London applied these principles to the hydrogen molecule in 1927. This breakthrough resulted in the explanation of the chemical bonding between hydrogen atoms. Since then, quantum chemistry has been critically important in the understanding of molecular properties such as stability, reactivity, and applicability for devices in various spheres.