"For his numerous fundamental contributions to the theory of rovibronic interactions in molecules, especially the development of the universally used Watson Hamiltonian for vibration-rotation energy levels, the unified treatment of centrifugal distortion in molecules, the elucidation of forbidden rotational transitions in spherical tops, the application of advanced symmetry arguments to perturbations in external fields, and investigations of the Jahn-Teller effect in H₃ and NH_4."