[lammps-users] would you please help me about Shake command

If you’re asking, can I fix just the bond lengths in a long-chain polymer,
then you can’t. LAMMPS doesn’t support that model. It’s not particularly
parallel to enforce that constraint. You can rigidify the entire molecule (fix rigid).
Or you can run with stiff springs, or you can run it with rRESPA to increase the
time scale, etc. But you can’t SHAKE just the bonds.