James Watson made important contributions to our understanding of the theory of molecular spectroscopy. His studies led to simplifications of the fundamental Hamiltonian, recognition of specific symmetry properties, the development of efficient perturbation procedures, and a deeper understanding of the isotopic dependence of molecular parameters. His examination of centrifugal distortion effects led to a reduction in the number of parameters for asymmetric top molecules, to the discovery of clustering in high levels of spherical tops, and to the prediction of forbidden rotational spectra in certain nonpolar molecules, including methane. His later work included the theory of the Jahn-Teller effect in the Rydberg states of H3 and D3.

Dr James Watson FRS died on 18 December 2020.