I have used from bond_style harmonic for TIP4P/ICE, while I do not have the number to k (spring constant in E=K(r-r.) . what number i should use?

I have used from bond_style harmonic for TIP4P/ICE, while I do not have the

number to k (spring constant in E=K(r-r.) . what number i should use?

any number you like.

It doesn't matter which value you choose. You only need to specify the bond so that fix shake knows which pairs of atoms to keep at fixed separation.