  Mijnarends et al. [J. Phys. Condens. Matter {\bf 10}, 10383 (1998)] contested
the best existing calculations of positron annihilation rates in jellium and
crystal lattices, pointing in this way at deficiencies of existing theories of
electron-positron interaction in these materials. In the present work the local
enhancement factors due to positron-electron interaction in Li, Be, B, C, N and
O are computed in a consequent many-body approach for core and as concerns
lithium also for conduction electrons and compared to the results of existing
approximations to this problem which avoid direct many-body calculations in
metals, i.e. the local density, generalized gradient and weighted density
approximations, as well as to experimental data. Conclusions about positron
lifetime and positron-electron correlation energy are also presented.
Suggestions concerning annihilation rates in an electron gas agree with those
of Mijnarends et al.
