Modifications to the Orthogonalized-Plane-Wave Method for Use in Transition Metals: Electronic Band Structure of Niobium

Modifications to the orthogonalized-plane-wave (OPW) method are employed to facilitate its application to transition metals. The procedure is to augment the basis set of OPW's by including functions which vanish in the interstitial regions of the crystal but represent well the outer core functions and the $d$-band states near the nuclei. The bands are found to converge at a rate approximately the same as for the unmodified OPW method in semiconductors. The method is applied to calculate the conduction electron bands of niobium, along principal symmetry directions, to a convergence of about 0.01 Ry. The resulting band structure is very similar to previous augmented-plane-wave calculations for other bcc transition metals.