Theoretical chemistry seeks to provide explanations to chemical and physical observations. Should the properties derived from the quantum theory give a good account of the above mentioned phenomena, we derive consequences using the same theory. Should the derived consequences fall too far from the experimental evidence, we go to a different theory. G. Lewis proposed that chemical properties originated from the electrons of the atom's valence shell, ever since the theoretical chemistry has dealt with modelling of the outer electrons of interacting atoms or molecules in a reaction. Theoretical chemistry includes the fundamental laws of physics Coulomb's law, Kinetic energy, Potential energy, the Virial Theorem, Planck's Law, Pauli exclusion principle and many others to explain but also predict chemical observed phenomena. The term quantum chemistry which comes from Bohr's quantized model of electron in the atom, applies to both the time independent Schrödinger and the time dependent Dirac formulations. This book is a valuable companion for instructors, course designers and students. It is suitable for direct applications in practical courses in theoretical chemistry and atomic physics as well as for teaching advanced features of EXCEL in IT courses.