Theoretical ionic models—the modified electron-gas approach

Theoretical ionic models—the modified electron-gas approach 

The details of the modified electron-gas (MEG) ionic model method have been fully described by Gordon and Kim (1972). The fundamental assumptions of the method are (1) the total electron density at each point is simply the sum of the free-ion densities, with no rearangements or distortion taking place (2) ion-ion interactions are calculated using Coulomb s law, and the free-electron gas approximation is employed to evaluate the electronic kinetic, exchange, and correlation energies (3) the free ions are described by wave functions of Hartree-Fock accuracy. Note that this method does not iterate to a self-consistent electron density. 

The Coulombic part of the interaction energy between ions a and b is given by the expression  

The MEG method seems to be of considerable general utility even for compounds in which one would expect considerable covalency, so long as the structure is relatively compact and does not involve important angular variables. For example, the geometries of main-group sulfides and nitrides are predicted fairly accurately by this method (Muhihausen and 

Gordon, 1981). However, for Si oxides the SS-MEG method yields Si-0-Si angles that are too large. Models incorporating oxygen polarization are currently being developed to deal with such deficiencies (Jackson and Gibbs, 1988). 

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