Local structure in water and its comparison with hexagonal ice from molecular dynamics simulations of TIP4P/2005 water model

Molecular simulations show that liquid water if sufficiently cooled can separate into low-density (tetrahedral) and high-density (non-tetrahedral) forms. We perform molecular dynamics simulations of TIP4P/2005 liquid water at 1 and 1000 bar and in the temperature range 280 to 200 K. Under these conditions we analyse liquid water's local structure and compare it with hexagonal ice. A spherical cut-off is used to collect nearest neighbours of oxygen atoms in the first coordination shell. Local orientational order parameters are then used to characterise the orientation in oxygen and hydrogen atoms in these molecules. At 1 and 1000 bar liquid water which is mostly 4 and 5 coordinated at 280 K becomes 4 coordinated when sufficieintly cooled. Oxygen atoms become more orientationally ordered with cooling and at temperatures less than 230 K its orientation is ice-like. Hydrogens tend to orient out-of-plane with cooling thus allowing the 4 coordinated water to form network like structures. Similar to oxygen ordering, hydrogens also prefer ice-like arrangment with supercooling.