Various experimental data obtained at the beginning of the 20th century could not be explained from the standpoint of classical chemistry. Such data include, for example, the spectra of gaseous substances, intensity distribution in the radiation spectra of solids, and the Compton effect. The search for explanations for these phenomena led to the emergence of a new field of knowledge. In turn, the precise quantum-mechanical calculations of molecular properties led to a revision of basic concepts used in chemistry to explain the formation of chemical bonds.

Today quantum chemistry gives answers to the following questions: What is the equilibrium geometry of a molecule? What is its energy? What are its characteristics (dipole moment, polarisability)? How fast are transitions between conformations? What is the dependence of molecular characteristics on time? How can several molecules interact? It also allows the position of absorption and luminescence bands, NMR and ESR spectra, and the form of vibrational absorption spectra, etc. to be predicted.

In quantum mechanics, any system can be described by the Schrodinger equation (or Dirac equation, in a relativistic case), whose solution allows specifying the wave functions of the system, which, in turn, can be used to calculate all the parameters characterising this state. However, the Schrodinger equation cannot be solved analytically, even for the simplest molecules. For that reason quantum chemistry is often called “computational chemistry”, as it uses different mathematical approaches for the numerical solving of equations, and requires a high computing capacity to implement these methods. The distribution of quantum chemistry methods is limited by the current state of development of modern computer technology. It is expected that with an increase in computing power, the sizes of objects whose state can be calculated using the methods of quantum chemistry will grow. Today there exist numerous computer programs implementing the methods of quantum chemistry. The most widespread of those are Gaussian and GAMESS.