Liquid water and biological systems: the most important problem in science that hardly anyone wants to see solved.

The main emphasis of this paper is the design of a water model that gives the correct temperature-dependent density. Water interaction models currently used have been highly oversimplified and are presently incapable of producing, over even a modest range of temperature and pressure, the properties of real water. The new feature is a modified second-neighbour non-hydrogen-bonded interaction to match those known to exist in the moderately dense ice polymorphs or the high density amorphous solid. Combined dynamically in the liquid with ordinary tetrahedral bonding, the more dense metastable structure tends to grow in with increasing temperature because of the entropic driving force, creating the density maximum. With this new model, more realistic structural and dynamic properties of liquid water near surfaces, particularly biologically important macromolecules and membranes, can be expected in future work.