Ice: A fruitful source of information about liquid water

The recent calculation by computer simulation of the phase diagram involving the solid phase of water has triggered a number of new simulation experiments with somewhat unexpected results. Firstly, it has been shown that this is a severe test of the potential models. The results yielded by TIP4P and derived models are, in general, in fair agreement with experiment. But the differences between the experimental phase diagram and the computer simulation for the three site models (SPC, SPC/E and TIP3P) and for the five site model TIP5P are significant. In this paper we show how this information can be useful to develop improved potentials. We discuss the difficulties in fitting different properties and analyze the reasons that led us to the proposal of two different models: TIP4P/Ice and TIP4P/2005. The first model was intended to reproduce the melting temperature of hexagonal ice while the second one was designed as a general model.