  Applying the generalization of the model for chain formation in
break-junctions [JPCM 24, 135501 (2012)], we study the effect of light
impurities on the energetics and elongation properties of Pt and Ir chains. Our
model enables us with a tool ideal for detailed analysis of impurity assisted
chain formation, where zigzag bonds play an important role. In particular we
focus on H (s-like) and O (p-like) impurities and assume, for simplicity, that
the presence of impurity atoms in experiments results in ..M-X-M-X-... (M:
metal, X: impurity) chain structure in between the metallic leads. Feeding our
model with material-specific parameters from systematic full-potential
first-principles calculations, we find that the presence of such impurities
strongly affects the binding properties of the chains. We find that while both
types of impurities enhance the probability of chains to be elongated, the
s-like impurities lower the chain's stability. We also analyze the effect of
magnetism and spin-orbit interaction on the growth properties of the chains.
