  The Infrared Spectrum is used as an experimental data target, to improved the
TIP4P/$\epsilon$, adding harmonic potential U(r) in all bonds and harmonic
potential U({\theta}) in the angle formed by the hydrogens and oxygen atoms of
the water molecule. The flexibility in the molecule gives the ability of the
water molecules to change, around different temperatures and pressures, their
structure and in the bulk liquid the distribution of the dipole moment. This
distribution helps to reproduce better the experimental data that the rigid
models can not describe. The rigid water models of 3 and 4 sites have
limitations to describe all the experimental properties, and is because can not
take on how the dipole moment is distributed of dipole moment around the
different thermodynamics phases. The new flexible TIP4P/$\epsilon$ Flex water
model is compared to the improve models of TIP4P and OPC model.
