  We study the stability and structure of self-assembled atomic chains on
Si(111) induced by monovalent, divalent and trivalent adsorbates, using
first-principles total-energy calculations and scanning tunneling microscopy.
We find that only structures containing exclusively silicon honeycomb or
silicon Seiwatz chains are thermodynamically stable, while mixed
configurations, with both honeycomb and Seiwatz chains, may be kinetically
stable. The stability and structure of these atomic chains can be understood
using a surprisingly simple electron-counting rule.
