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Axel D. Becke develops practical approximations for density functional theory. His 1988 exchange functional uses the local electron density and its gradient to improve on a purely local density approximation. Its construction imposes the proper long-distance behavior of the exchange-energy density and fits one parameter using atomic exchange energies. This supplies a concrete example of building a useful approximate energy functional from physical constraints and reference calculations. The asymptotic statement concerns an energy density, and does not automatically establish the asymptotic behavior of its functional derivative.^[ref]