Physical grounds for the DFT approximations

The LDA is not as primitive as it looks. The electron density distribution for the homogeneous gas model satisfies the Pauli exclusion principle and, therefore, this approximation gives the Fermi holes that fulfil the boundary conditions with eqs. (11.62), (11.63) and (11.64). The LDA is often used because it is rather inexpensive, while still giving reasonable geometry of molecules and vibrational frequencies. The quantities that the LDA fails to reproduce are the binding ener-gies, ionization potentials and the intermolecular dispersion interaction. 

Perdew noted a really dangerous feature in an innocent and reasonable looking GEA potential. It turned out that in contrast to the LDA the boundary conditions for the electron holes were not satisfied. For example, the exchange hole was not 

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