Number 259 (Story #3), February 21, 1996 by Phillip F. Schewe and Ben Stein ONE OF THE GREAT MYSTERIES OF WATER , its tendency to shrink when warmed, has been successfully modeled for the first time. As cold water is heated, it reaches a minimum volume--and therefore a maximum density--at around 4 degrees Celsius. No theoretical model has been able to explain this "density anomaly." This shortcoming compromises the accuracy of molecular-scale models of proteins and other systems involving water. Now, researchers at Texas Tech University (G. Wilse Robinson, 806- 742-3099) have proposed an explanation for the density anomaly by looking beyond neighboring molecules in the liquid and focusing on more distant "second neighbors." In all ten known forms of ice, an H20 molecule is surrounded by its closest neighbors in the same way. However, in low-density ice, second-neighbor molecules are relatively distant from the central molecule, while in some of the higher-pressure, dense forms of ice, oxygens belonging to second- neighbor molecules bend around to form a closer approach to the central molecule. The researchers propose that in the liquid these "bent" second neighbors would have somewhat less stability than those in the low-density arrangement and therefore the denser bent neighbors would form only at warmer temperatures. Using a simple model in which an oxygen atom and its second neighbors arrange themselves on a one-dimensional array, the Texas Tech researchers obtained density curves with a temperature and pressure behavior similar to that of water. (C.H. Cho et al., Physical Review Letters, 5 March 1996.)