Robert O. Jones
Abstract A practical definition of "strongly correlated" systems covers those that cannot be described well by density functional (DF) theory. DF theory has become an indispensable part of computational condensed matter physics and chemistry, but its origins go back to the early years of quantum mechanics in the late 1920s, and this chapter is devoted to a historical view of its development. Thomas and Fermi recognized the electron density as a basic variable, and Dirac showed already in 1930 that the state of an atom is completely determined by its density; it is not necessary to specify the wave function. We follow the development of these ideas in the following decades to the single-particle equations of Kohn and Sham in 1965. Many details of the history are not well known, even to specialists in the field. The single application discussed is the Be2, which was perhaps the first unexpected DF prediction on small molecules that proved to be correct.
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