water models

hello dear users
why we ues the chare of -0.83 for O and 0.415 for H in Tip3p & Tip4p models? (we know that O is 2- & H is 1+)

If you're asking why Section 6.7 of the LAMMPS manual,
tells you to use non-integer partial charges on the O and H in TIP3P,
it's because that is the definition of the model. See the
original papers for an explanation of why.