Positron Annihilation and Orthogonalized Plane Waves in Lithium

Anisotropies in the momentum distribution of positron annihilation radiation have in the past been observed in several single-crystal metals, most recently in Li. Interpretation of the results in terms of Fermisurface structure is complicated. The angular correlation pattern is affected not only by the Fermi-surface shape but also by the necessary departure of the conduction-electron wave function from a plane wave. The magnitude of these two effects is evaluated in Li. The orthogonalized-plane-wave method with 19 waves is used to obtain the lowest band, $E\left(\mathrm{k}\right)$, and the corresponding conduction-electron wave functions. The numerically calculated angular correlation curves show anisotropies in agreement with the measured results if both the effect of the Fermi-surface shape and the effect of the wave function are included. If the Fermi-surface shape alone is considered, the observed anisotropies cannot be explained. Quantitative agreement with experiment can be obtained if, in addition to all other effects, the enhancement factor due to electron-positron attraction calculated by Kahana is included.