He ultimately held a position jointly in both the physics and chemistry departments. At both NYU and Chicago, he continued to refine his molecular-orbital theory. Up to this point, the primary way to calculate the electronic structure of molecules was based on a calculation by Walter Heitler and Fritz London on the hydrogen molecule (H2) in 1927. With the conception of hybridized atomic orbitals by John C. Slater and Linus Pauling, which rationalized observed molecular geometries, the method was based on the premise that the bonds in any molecule could be described in a manner similar to the bond in H2, namely, as overlapping atomic orbitals centered on the atoms involved. Since it corresponded to chemists' ideas of localized bonds between pairs of atoms, this method (called the Valence-Bond (VB) or Heitler-London-Slater-Pauling (HLSP) method), was very popular. In attempting to calculate the properties of excited states (molecules that have been excited by an energy source), the VB method does not always work well. With its description of the electron wave functions in molecules as delocalized molecular orbitals that possess the same symmetry as the molecule, Hund and Mulliken's molecular-orbital method, including contributions by John Lennard-Jones, proved to be more flexible and applicable to a vast variety of types of molecules and molecular fragments, and has eclipsed the valence-bond method. As a result of this development, he received the Nobel Prize in Chemistry in 1966. Mulliken became a member of the National Academy of Sciences in 1936, the youngest member in the organization's history at the time. He was elected a Foreign Member of the Royal Society (ForMemRs) in 1967. Mulliken population analysis is named after him, a method of assigning charges to atoms in a molecule. Personal life On December 24, 1929, he married Mary Helen von Noé, daughter of Adolf Carl Noé, a geology professor at the University of Chicago. They had two daughters. Later years In 1934, he derived a new scale for measuring the electronegativity of elements. This does not entirely correlate with the scale of Linus Pauling, but is generally in close correspondence. In World War II, from 1942 to 1945, Mulliken directed the Information Office for the University of Chicago's Plutonium project. Afterward, he developed mathematical formulas to enable the progress of the molecular-orbital theory. In 1952 he began to apply quantum mechanics to the analysis of the reaction between Lewis acid and base molecules.