Further developments in the local-orbital density-functional-theory tight-binding method

Improvements to the Sankey-Niklewaki method O. F. Sankey and D. J. Niklewski, Phys. Rev. B 40, 3979 (1989) for computing total energies and forces, within an ab initio tight-binding formalism, are presented here. In particular, the improved method (called FIREBALL) uses the separable pseudopotential (Hamann or Troullier) and goes beyond the minimal sp^2 basis set of the Sankey-Niklewski method, allowing for double numerical basis sets with the addition of polarization orbitals and d orbitals to the basis set. A major improvement includes the use of more complex exchange-correlation functionals, such as Becke exchange with the Lee-Yang-Parr correlation. Results for Cu and GaN band structures using d orbitals within the improved method are reported; the results for GaN are greatly improved compared to the minimal basis results. Finally, to demonstrate the flexibility of the method, results for the H₂O dimer system and the energetics of a gas-phase octahydro-1, 3, 5, 7-tetranitro-1, 3, 5, 7-tetrazocine molecule are reported.