Gradient correction for positron states in solids

First-principles calculations of positron-annihilation characteristics in solids are usually based on the local-density approximation (LDA) for positron-electron correlation. The LDA systematically overestimates the annihilation rate. As a remedy we introduce a generalized gradient approximation (GGA). Our results for several metals and semiconductors show that the GGA systematically improves the predictive power of positron lifetime calculations over those based on the LDA. We compare also the resulting positron energy levels in solids with data from slow-positron experiments.