In 1926, the Austrian physicist Erwin Schrödinger (1887-1961) made a fundamental mathematical discovery that had a profound impact on the study of the molecular world (in 1933, Schrödinger was awarded the Nobel Prize in Physics just seven years later, his breakthrough discovery). He discovered that a quantum system’s state composed of particles (such as electrons and nucleons) could be described by postulating the existence of a function of the particle coordinates and time, called state function or wave function (, psi function). This function is the solution of a wave equation: the Schrödinger equation (SE). Although the SE equation can be solved analytically only for relatively simple cases, the development of computer and numerical methods has made possible the application of SE to study complex molecules.