Computational quantum chemistry is one of the most successful techniques to calculate the main properties of molecules and solids. It is also widely used in the design of new pharmaceutical drugs and biological materials. In general, it is based entirely on quantum mechanics and its basic physical constants, the so-called ab initio method. Usually, the unveiling of the DNA molecule, as well as the secondary structure (alpha-helix and beta-sheet) in proteins, in the 1950s, together with other relevant breakthrough discoveries, marked its dawn. Since then, many achievements in biology, chemistry, physics, and pharmaceutical science were obtained, leading to its solid reputation as a primer tool now available to scientists. It allows investigations with greater accuracy and at an unprecedented level of detail, leading not only to the ability to make a direct comparison with experimental data but also even to predict hitherto unobservedimportant phenomena.