# Quantum chemistry

The **quantum chemistry** is the application of quantum mechanics to chemical problems such. B. the description of the electronic structure of atoms and molecules and the effects on their reactivity and thus a sub-area of theoretical chemistry . (Quantum mechanical studies of *atoms* are seen as the boundary line between chemistry and physics and are not necessarily assigned to quantum chemistry.) The basis for most quantum chemical methods is the Schrödinger equation . However, since this can only be solved for very simple systems within the Born-Oppenheimer approximation , further approximations must be introduced.

The hydrogen molecule was one of the first to be investigated and calculated using quantum chemistry, in 1927 by the German scientists Walter Heitler and Fritz London . The method they developed was extended by the American chemists John C. Slater and Linus Pauling to the Valence Bond (VB) or *Heitler-London-Slater-Pauling (HLSP) method* . In it, the focus is placed on the consideration of the pairwise interaction between atoms, so it fits well with the classic consideration of the chemical bond .

An alternative approach to the nature of chemical bonding was developed by Friedrich Hund and Robert S. Mulliken , which describes electrons as delocalized in the form of mathematical functions . The description, known as the *Hund-Mulliken* or, more commonly, the Molecular Orbital (MO) method, is less intuitive for the classical chemist, but is now by far more common.

With both approaches, mathematical approximation methods can be used to predict material properties.

The applicability of quantum chemistry in the form of computer calculations can be found under computational chemistry .

Some further topics are:

- the Hartree-Fock method (SCF method (SCF: self-consistent field))
- post-HF methods (e.g. Configuration Interaction )
- the density functional theory
- Semi-empirical methods

In addition, the following people are important for the development of quantum chemistry:

- Erich Hückel
- Hans Hellmann
- Rudolph Pariser
- Robert G. Parr
- John Pople
- Henry Eyring ( Eyring Theory )
- Reinhart Ahlrichs
- Werner Kutzelnigg

## literature

- Kostas Gavroglu, Ana Simões:
*Neither Physics nor Chemistry. A History of Quantum Chemistry.*MIT Press, 2011, ISBN 0-262-01618-4 - F. Jensen:
*Introduction to Computational Chemistry*. John Wiley & Sons, Chichester 1999, ISBN 978-0-471-98425-2 . - Attila Szabo, Neil S. Ostlund:
*Modern Quantum Chemistry: Introduction to Advanced Electronic Structure Theory,*Dover Publications Inc., 1996, ISBN 978-0486691862

## Web links

- QMC @ home BOINC project (distributed computing) at the University of Münster on the quantum Monte Carlo method