Thomas and Fermi studied the homogeneous electron gas in the early 1920’s
[12]. The orbitals of the system are, by symmetry, plane waves. If the electronelectron interaction is approximated by the classical Hartree potential (that is
exchange and correlation effects are neglected) then the total energy functional can be
readily computed [ 12]. Under these conditions the dependence of the kinetic and
exchange energy ( Equation 7) on the density of the electron gas can be extracted
(Dirac [ 13,1,14]) and expressed in terms of a local functions of the density. This
suggests that in the inhomogeneous system we might approximate the functionalas an
integral over a local function of the charge density. Using the kinetic and exchange
energy densities of the non-interacting homogeneous electron gas this leads to
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