Part 1. Background Material In this portion of the text, most of the topics that are appropriate to an undergraduate reader are covered. Mamy of these subjects are subsequently discussed again in Chapter 5, where a broad perspective of what theoretical chemistry is about offered. They are treated again in greater detail in Chapters 6-8 where the three main disciplines of theory are covered in depth appropriate to a graduate-student reader Chapter I. The Basics of Quantum Mechanics Why Quantum Mechanics is Necessary for Describing Molecular Properties. We know that all molecules are made of atoms which in turn contain nuclei and electrons. As I discuss in this introductory section, the equations that govern the motions of electrons and of nuclei are not the familiar Newton equations F=m a but a new set of equations called Schrodinger equations. When scientists first studied the behavior of electrons and nuclei, they tried to interpret their experimental findings in terms of classical Newtonian motions, but such attempts eventually failed. They found1 Part 1. Background Material In this portion of the text, most of the topics that are appropriate to an undergraduate reader are covered. Many of these subjects are subsequently discussed again in Chapter 5, where a broad perspective of what theoretical chemistry is about is offered. They are treated again in greater detail in Chapters 6-8 where the three main disciplines of theory are covered in depth appropriate to a graduate-student reader. Chapter 1. The Basics of Quantum Mechanics Why Quantum Mechanics is Necessary for Describing Molecular Properties. We know that all molecules are made of atoms which, in turn, contain nuclei and electrons. As I discuss in this introductory section, the equations that govern the motions of electrons and of nuclei are not the familiar Newton equations F = m a but a new set of equations called Schrödinger equations. When scientists first studied the behavior of electrons and nuclei, they tried to interpret their experimental findings in terms of classical Newtonian motions, but such attempts eventually failed. They found