  Several first-principles calculations of positron-annihilation
characteristics in solids have added gradient corrections to the local-density
approximation within the theory by Arponen and Pajanne [Ann. Phys. (N.Y.) 121,
343 (1979)] since this theory systematically overestimates the annihilation
rates. As a further remedy we propose to use gradient corrections for other
local density approximation schemes based on perturbed hypernetted-chain and on
Quantum Monte Carlo results. Our calculations for various metals and
semiconductors show that the proposed schemes generally improve the positron
lifetimes when they are confronted with experiment. We also compare the
resulting positron affinities in solids with data from slow-positron
measurements.
