Dipole-Quadrupole Force Ratios Determine the Ability of Potential Models to Describe the Phase Diagram of Water

We show that, in order to yield an acceptable phase diagram for water, the key ingredient is the relative strength of the dipolar and quadrupolar forces. We also demonstrate that for three-point-charge water models the relative strength of the dipolar to quadrupolar forces is determined by the position of the negative charge. This provides a theoretical background--supported by the experiment--to our knowledge of the water interactions which at present is essentially dominated by empirical models. In addition, it can be quite useful in the development of improved water models with a more physical basis.