Friedrich Hund and Chemistry

Friedrich Hund, one of the few physicists still alive of the generation that pioneered quantum mechanics, celebrates his 100th birthday this year. This is an occasion to recall how much chemistry owes to him. The important contributions of Hund to theoretical chemistry will not be presented in a primarily historical context—the cooperation of Hund and Mulliken during the development of MO theory will, however, be duly discussed—but the stress will be on the impact of the concepts introduced by Hund on theoretical chemistry today. Nearly all methods of quantum chemical calculations presently in use have a first step based on Hund's MO theory. Hund recognized that use of delocalized molecular orbitals does not necesarily conflict with a description of a molecule in terms of localized bonds. Moreover, he formulated conditions for localizability that are still pertinent today. The classification of the spectra of the spectra diatomic molecules by Hund in 1925 was virtually definitive. Recently Hund's rules, which, among other things, make predictions about the energetic ordering of states with different spin multiplicities within the same electronic configuration, have aroused renewed interest. The concept of atomic states of natural and unnatural parity plays a key role in this context. Hund's first tow rules for atomic states have been found to be special cases of more general rules. Molecules can violate Hund's first rule though spin polarization and though the preference of different equilibrium geometries of the singlet and triplet states.