In 1952 he began to apply quantum mechanics to the analysis of the reaction between Lewis acid and base molecules. (See Acid-base reaction theories.) He became Distinguished Professor of Physics and Chemistry in 1961 and continued in his studies of molecular structure and spectra, ranging from diatomic molecules to large complex aggregates. In 1981, Mulliken became a founding member of the World Cultural Council. In 1983, Mulliken received the Golden Plate Award of the American Academy of Achievement. He retired in 1985. His wife died in 1975.