This book discusses major developments of quantum mechanics from classical to computational chemistry. The book is student and user friendly and includes exhaustive derivations, mathematical proofs, and theorems. A series of solved numerical have been added after each topic to have a better understanding of the subject. It will be helpful to Chemistry students at undergraduate and postgraduate level, as well for those appearing in various competitive examinations. Print edition not for sale in South Asia (India, Sri Lanka, Nepal, Bangladesh, Pakistan or Bhutan)