Density Functional Theory for Solids, Surfaces, and Molecules: From Energy Bands to Molecular Bonds

In the 1950’s, Slater introduced the concept of the electron-gas approximation as a simplification of the Hartree-Fock equations (Slater, 1951). The goal was to make energy band structure calculations practical for crystalline solids such as bulk Cu. The exchange term of the Hartree-Fock equations needed to be approximated while the kinetic energy and the Coulomb repulsion terms of the Fock operator could be treated rather well within a periodic lattice, particularly if the so-called muffin-tin approximation (Slater, 1937) was made.