Combined quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations have been carried out to study liquid water. The QM forces are evaluated at the Hartree-Fock level. The QM/MM coupling potentials, constructed from the flexible TIP3P Lennard-Jones parameters, and from those modified according to the corresponding QM/MM calculations of the water dimer, are examined based on the structure of liquid water, polarization effects of the QM water molecule from the surrounding classical MM water molecules, and the interactions between the QM molecule and the MM molecules. Our simulations show that when the flexible TIP3P Lennard-Jones parameters are used, the QM/MM coupling is too strong. However, when the Lennard-Jones parameters on the QM water molecule are modified according to the corresponding QM/MM calculations of the water dimer, the coupling between the QM water molecule and MM water molecules becomes too weak. In general, our work shows that the Lennard-Jones parameters on the QM atoms have a very large effect on the combined QM/MM simulation results.