I read the chain.py, which is used to create bread-spring chains for lammps input, but it is only for linear polymer chains, I want
to ask how to create branched polymer chains, such as three arm polymers. Thanks in advance.
If you mean the chain.py tool in Pizza,py, then it is pretty simple.
You can modify it if you think you know the rules for how you
want to generate branching polymers. If not, then I suggest you
look at other builders. E.g. see those listed on the Pre-Post Processing
link of the LAMMPS web site.