where and are the electron and positron chemical potentials, respectively [27]. In the case of a semiconductor, is taken from the position of the top of the valence band. The affinity can be measured by positron re-emission spectroscopy [28]. The comparison of measured and calculated values for different materials is a good test for the electron-positron correlation potential. The Ps atom work function is given by [28]

Since the Ps is a neutral particle, eps is independent of the surface dipole. The

Figure 5. Positron affinities for several metals. The solid and open circles give the GGA and LDA results as a function of the experimental ones, respectively. The solid line corresponds to the perfect agreement between the theoretical and experimental results from refs. [28].

LDA shows a clear tendency to overestimate the magnitude of A+ [19]. This overestimation can be traced back to the screening effects. In the GGA, the value of A+ is improved with respect to experiment by reducing the screening charge. The calculated positron affinities within LDA and GGA against the corresponding experimental values for several metals are shown in Fig. 5. Kuriplach etal. [29] calculated A+ for different polytypes of SiC and showed that the GGA agrees better with the experimental values than the LDA. Panda et al. showed that the computed affinities depend crucially on the electron-positron potential used in the calculation (LDA or GGA) and on the quality of the wave function basis set [30]. The result with a more accurate basis set for valence electrons and within GGA gives -3.92 eV for 3C-SiC, which is surprisingly close to the experimental [30].

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