How to build an infinite long chain by using the periodic boundary condition?

Dear all,

Is there any method to build an infinite long chain in LAMMPS? For example, the chain is connected only by harmonic bonds (not non-bonded interactions), that can I use the periodic boundary condition to connect the two ends of the chain?

This question comes up every once in a while in this list. Search the mailing list (or just google). See for example:
http://lammps.sandia.gov/threads/msg33980.html