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The authors describe a fully relativistic augmented-plane-wave (APW) method where the basis functions are pure spin functions in the large component. This feature allows spin-mixing interactions to be separated and treated more efficiently than in the standard relativistic APW method. These basis functions are constructed by solving an approximate relativistic radial equation. In addition, the energy derivative is used in the construction of the basis functions so that one obtains adequate variational freedom solving a linear secular equation. Both the utility and the limitations of the technique are discussed.
MacDonald et al. (Tue,) studied this question.