GeAs, atomic density

Other semiconductors, such as Ge and GeAs, have atomic densities of 4.4 X 10 atoms/cm, a metal such as aluminum has about 6 X lO atoms/cm, and metals such as cobalt, nickel, and copper have about 9 X lO atoms/cm. ... 

The approximations most often employed in atomic and molecular studies are the local density approximation (LDA), in which the energy density is a function of the density only, and its extension, the gradient expansion approximation and/or generalized gradient approximation (GEA/GGA), where besides the local density, the nonlocal density gradient is included in the energy density function formulation that is,... 

Numerical tests of these gradient expansions for atoms show that the second-order gradient term provides a useful correction to the Thomas-Fermi or local density approximation for Tg, and a modestly useful correction to the local density approximation for E, but seriously worsens the local spin density results for E and E xc. In fact, the GEA correlation energies are positive The latter fact was pointed out in the original work of Ma and Brueckner [69], who suggested the first generalized gradient approximation as a remedy. 

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