How was Robert S. Mulliken awarded the Noble Prize?

Name: Robert S. Mulliken. 
Country: American.
Date of Birth: 7 June 1896.
Date of Death: 31 October 1986.
The subject of Study: Chemical bonding, molecular orbital theory.
Noble Prize Year: Nobel Prize for Chemistry in 1966 for his research fundamental work concerning chemical bonds and the electronic structure of molecules by the molecular orbital method.