LAUSR

Abstract The Infrared Spectrum is used as an experimental data target to improved the TIP4P/ ∊ force field of water by adding harmonic potential U(r) in all bonds and harmonic potential U( θ ) in the angle formed by the hydrogens and oxygen atoms of the water molecule to give freedom of movement through flexibility. The flexibility in the water molecule, gives it the ability to change its structure around different temperatures and pressures, which describes a distribution of the dipole moment in each thermodynamic state. 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 around the different thermodynamics phases. The new flexible TIP4P/ ∊ Flex water model is compared, in different thermodynamic conditions in gas, liquid and solid phases, to the improve models of TIP4P (TIP4P/05, TIP4P/ ∊ , TIP4P-FB, TIP4P-ST), as well OPC (four sites) and with the recent flexible model of water of three sites FAB/ ∊ model, obtaining a better evaluation with respect to the mentioned models.