 Among the most important were his discovery of Geometric phase at the conical intersection of potential energy surfaces, his introduction of the correlation diagram approach to the study of Woodward-Hoffmann rules, and his introduction of nuclear permutation-inversion symmetry groups for the study of molecular symmetry.In his later years at Cambridge he became interested in the brain and the new field of artificial intelligence.As a consequence, in 1967, he made a major change in his career by moving to the University of Edinburgh to co-found the Department of Machine intelligence and perception, with Richard Gregory and Donald Michie.