Stretched water, simulate water for different densities

So--called stretched water is water at a density lower that at normal room temperature. Experimentally one can stretch water by placing it in a cylinder with a tight fitting piston and pulling the piston outward without letting air into the system. This experiment is hard to do, because an immense force is needed to expand the system significantly.

In contrast it is easy to simulate stretched water using the Wasser program. Simply specify a density less than using the NVT ensemble or set the pressure to a value smaller than 62 Mpa in the NPT ensemble.

Background :

When looking at molecules of water at density we see a randomly structured hydrogen bond network. When we start decreasing the density, the pressure of the system starts to decrease and gradually takes negative values.

To understand negative pressure we can picture a rubber band that we hold between our fingers. When we stretch the rubber band, it tries to snap back to its relaxed position. We can say that the ``outward force of the rubber band on our fingers is negative.'' The same thing happens when we put water into a closed container without bubbles then stretch the container. Both the container and the water try to snap back to their relaxed conformation. ``Pressure'' ordinarily means ``outward force per unit area.'' In this sense, the pressure in the water goes negative.

Stretching the water makes a greater volume available to the molecules. The stretched network strives for the energetically most favoured conformation. This conformation is closely related to that of ice, since a water molecule now, due to the tension, has fewer nearest neighbors than it had at density . Under stretching, the system will gradually go from a random network to the geometrically favoured open hexagonal structure as the density is decreased. If the density is lowered too far, however, the system reaches its mechanical stability point; any further reduction in density will ``tear the system apart.'' This occurs at a density of approximately . The system will no longer be in the liquid phase, it will be like a droplet in vacuum.

System settings :

Start with the NVT ensemble. Fix the temperature at 273 K and the density at . Now slowly lower the density.

Questions :

• What can be said about the hydrogen bond network when we reduce the density of water?

  the dynamics of the system?

  the orientation of the molecules?

• Liquid water cannot be stretched indefinitely; there is a mechanical stability point. If this point is exceeded, the system no longer represents a liquid, rather it is transformed to a gaseous system. Find the mechanical stability point and describe what happens at this point.