Dear teacher,thanks again for your help.I have read your reply.But also have some questions.How to add one thermal atom of type 2 to each type 1 atom, attached by a spring. I’ve already used the command of fix spring/self .However, I did not get the desired results, and I judged this command was unreasonable. Which command can be implemented in the LAMMPS. According to my own understanding, I drew a picture.See Fig1.

Fig 1
I have find the command in the LAMMPS,fix 00 lower1 spring couple lower2 100 NULL NULL 10 0.0. I want to kown what the command meaning and whether I should to use it. I have to try in my code,but two group atoms are not move together. Please tell me how to achieve my ideal. Thanks a lot !