Quantum chemistry is the application of quantum mechanics to problems in chemistry.
The description of the electronic behavior of atoms and molecules as pertains to their reactivity is an application of quantum chemistry.
Since quantum-mechanical studies on atoms are considered to be on the borderline between chemistry and physics, and not always included in quantum chemistry, what is often considered the first true calculation in quantum chemistry was that of the German scientists Walter Heitler and Fritz London (though Heitler and London are generally classed as physicists) on the hydrogen (H2) molecule in 1927. Heitler and London's method was extended by the American chemists John C. Slater and Linus Pauling to become the Valence-Bond (VB) [or Heitler-London-Slater-Pauling (HLSP)] method. In this method, attention is primarily devoted to the pairwise interactions of atoms, and this method therefore correlates closely with classical chemists' drawing of bonds between atoms.
An alternative approach was developed by Friedrich Hund and Robert S. Mulliken, in which the electrons are described by mathematical functions delocalized over an entire molecule. The Hund-Mulliken approach [or molecular orbital (MO) method] is less intuitive to chemists, but since it turns out to be more capable of predicting properties than the VB method, it is virtually the only method used in recent years.
Some topics in quantum chemistry include:
Some people (other than those mentioned above) significant in the development of quantum chemistry: