This classic text, now in its fourth edition, has been updated and enlarged to reflect the latest developments in Quantum Chemistry. The book retains its emphasis on concepts and presents simplified mathematical treatment. It includes 109 problems of which 43 are new to this edition. Key Features: Includes new sections on projection operator, density matrix, tunneling and its applications, Hartree-Fock treatment of atoms, restricted and unrestricted Hartree-Fock theories and electron correlation. Covers state-of-the-art semi-empirical and abinitio molecular orbital methods to explain current methods of performing electronic structure calculations. Illustrates Born-Oppenheimer approximation and John-Teller effect with examples and carefully chosen problem. The book will essentially serve as a basic test for students of Quantum Chemistry at the postgraduate level. It will also be useful to students of Chemical Physics and to researchers in Physical Chemistry.