Total-energy full-potential linearized augmented-plane-wave method for bulk solids: Electronic and structural properties of tungsten

The development of the all-electron full-potential linearized augmented-plane-wave (FLAPW) method for bulk solids is reported. As in the thin-film FLAPW approach, the bulk FLAPW method solves the Kohn-Sham equations for a general charge density and potential. The formalism of Weinert, Wimmer, and Freeman for determining highly accurate total energies of solids within density-functional theory is implemented with all the necessary terms obtained from the FLAPW energy-band calculation. The resulting total-energy FLAPW approach is used to obtain highly accurate total-energy curves for bcc and fcc tungsten from which a number of structural properties (lattice parameters, bulk moduli, etc.) are derived. Calculated total energies have a relative precision of 0.1 mRy; a difference of 34 mRy is found between the (stable) bcc and fcc phases. The use of a simple quadratic form near the equilibrium value of the atomic volume is shown to lead to relatively large errors for the bulk modulus. Finally it is shown that in this all-electron method, all numerical approximations are controlled in that their effects can be minimized. One can therefore conclude that the FLAPW method is very well suited for testing the quality of various implementations of density-functional theory.