David P. Craig

Born December 23, 1919 in Sydney, Australia.

Formerly President Australian Academy of Science, and Emeritus Professor Australian National University.

AO, Fellow Royal Society, FAA.
Hon D.Sc. (Sydney), (Bologna).

Author of:

Important Contributions:

• Electronic Theory of aromatic molecules: The first application (1950) of configuration interaction to the molecular orbital theory of polyatomic molecules became an accepted feature of the theory. Problems in the ordering of the low excited states of benzene were resolved. Its use in cyclobutadiene led to the recognition of pseudo-aromatic systems sometimes later called anti-aromatic, such as pentalene, in which a set of alternating double bonds in a cyclic molecule is not a stabilizing feature of the electronic structure, and can be destablising. Later studies with Dr. N.L. Paddock were on aromatic-type stability in cyclic systems with dp orbitals together with pp in the phosphazene and thiazyl series. The new theoretical insight aided later experimental development of commercially important substances.
• Molecular Quantum Electrodynamics: The application of the non-covariant, Coulomb gauge, multipolar form of quantum electrodynamics led in work mainly with T. Thirunamachandran to more powerful methods for dealing with interactions between molecules, especially at large distances. The new viewpoint was successfully applied to chiral discrimination, namely the difference in forces between one optically active molecule and another according to whether the pair are dextro-dextro (or laevo-laevo) or dextro-laevo.
• Theory of the spectra of molecular crystals: Extension of Davydov's theory to include the coupling by crystalline interactions different free-molecule transitions. The theory correctly explains crystalline intensity transfers and enabled understanding, sometimes quantitative, of the intensities of the polarized spectral components.
• Vibronic interactions: The first quantitative account of spectral intensity “stealing” was given in an early paper of Craig's (1950); also the first application to a polyatomic molecule of the Franck Condon principle, for the purpose of calculating the change in molecular dimensions on electronic excitation. The estimate was used as the best available for many years.