A self-consistent field (SCF) procedure used to find approximate wave functions and energy levels in many-electron atoms. This procedure was introduced by the English mathematician and physicist Douglas Hartree in 1928 and improved by the Soviet physicist Vladimir Fock in 1930 (by taking into account the Pauli exclusion principle). The initial wave functions can be taken to be hydrogenic atomic orbitals. The resulting equations can be solved numerically using a computer. The results of the Hartree-Fock theory are sufficiently accurate to show that electron density occurs in shells around atoms and can be used quantitatively to show chemical periodicity.