It is shown that a generalization of the covalent bond energy of the tight-binding bond model to the case of a non-orthogonal basis set is an appropriate tool to describe the bonding properties of solids in a chemical language. It does not suffer from problems related to the ill-defined average electrostatic potential in periodic systems, in contrast to the formerly proposed crystal orbital Hamilton population (COHP). The new tool is applied to discuss the stability of the bcc, fcc and hcp structures of Nb, Mo, Ru and Rh.
We present the results of a calculation for the bulk electronic structure of gallium nitride in the zincblende phase. We determine the equilibrium lattice constant, the cohesive energy and the bulk modulus in the Density Functional approach within the Local Density Approximation (DFT-LDA). The one-particle eigenvalues of the DFT Kohn-Sham equation do in principle not agree with the experimental band structure. Therefore, we calculate the quasi-particle energies by including self-energy corrections to the DFT-LDA exchange correlation potential, with the GW approximation for the electron self-energy. We use norm-conserving pseudopotentials and a large plane-wave basis set (100 Ry cutoff) for a converged calculation in the DFT-LDA. The LDA band gap turns out to be very sensitive to the crystal volume. We fmd that GW corrections to the LDA band gap are significant. A detailed comparison with other DFT-LDA results and approximate GW calculations and with existing experimental data is given.
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