Efficientab initiotight binding

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Abstract
A fully ab initio tight-binding formalism, based on that of Sankey and Niklewski [O. F. Sankey and D. J. Niklewski, Phys. Rev. B 40, 3979 (1989)] is presented. Several modifications are proposed. In particular the question of basis sets and the effect of self-consistency have been given attention. It is found that double numeric basis sets give well-converged results, and that charge transfer need only be considered in the extreme case of bonding between atoms of widely differing electronegativities. Geometries are well described by the spherical atomic charge approximation. To obtain the maximum computational efficiency the number of lookup tables has been reduced to a minimum through the use of separable pseudopotentials and a many-center expansion for the exchange-correlation terms. This method is applied to the study of the effect of the temporary attachment of an electron to a hydrocarbon molecule and a fluorocarbon molecule.