  We present electronic structure and transport calculations for hydrogen and
lithium chains, using density functional theory and scattering theory on the
Green's function level, to systematically study impurity effects on the
transmission coefficient. To this end we address various impurity
configurations. Tight-binding results allow us to interpret our the findings.
We analyze under which circumstances impurities lead to level splitting and/or
can be used to switch between metallic and insulating states. We also address
the effects of strongly electronegative impurities.
