Invalid mass value

Dear stefan ,
Thanks for your replay.
Actually i want to make Tip4p model ‘s data file and i need to consider the dummy atom (M) for it.
Is it acceptable to consider a small number as M’ mass ?

When working with the TIP4P model in LAMMPS, there is no need to explicitly include the dummy particle positions; the code automatically computes and stores the positions of the dummy particles for you, reusing them where needed in the calculations of forces and energies.