To Anders Blom:
Thank you very much! So if I want to fix atoms in the central region, I need to know the ordinal number of atoms. By the way, the ordinal number of atoms are seperated by comma. For example, I want to fix the number of atoms (40,41,42,43,44,45), so I need to write 40,41,42,43,44,45 in "Geometric constraints". Is it right?