The Nobel Prize in Chemistry 1966 was awarded to Robert S. Mulliken "for his fundamental work concerning chemical bonds and the electronic structure of molecules by the molecular orbital method".
 

By chemical bond we mean the forces that tend to keep together the atoms in a molecule. in 1812 Berzelius suggested that these forces originate from positive and negative electrical charges of the atoms. In 1916 this discovery inspired Lewis to the hypothesis that the chemical bond is caused by two electrons, paired somehow and staying in the domain between the bonded atoms. In an epoch-making investigation in 1927 Heitler and London also succeeded in casting Lewis' pair theory in a physically more satisfactory form by aid of quantum mechanics. Professor Mulliken, it is now more than forty years since he started to investigate what the electron is doing in the molecule. His deep understanding of the physical laws, governing the behaviour of the electron, combined with his intimate knowledge of the problems of Chemistry have greatly advanced our understanding of the properties of molecules.