The present manuscript aims at gathering the modeling calculation tools available in the GAUSSIAN software, which are essential for the understanding of quantum chemistry. Simple examples of molecules, reactions, reaction mechanisms have been treated in order to focus the practical work on the methods and techniques of calculations. All the data necessary to solve the problems of chemical reactivity, geometry of molecules, etc. have been presented in this book. Finally, I hope that this manuscript will be a serious addition to the learning and success of the students.