Water structure can be characterized using statistical analysis of irregular polyhedra obtained as a result of a specific tessellation in three-dimensional point space. The Voronoi tessellation partitions the space into convex polytopes called Voronoi polyhedra. The Voronoi polyhedron is the region of space around an atom, such that all points of this region are closer to this atom than to any other atom of the system. A group of four atoms, whose Voronoi polyhedra meet at one vertex, forms another basic object of statistical geometry, the Delaunay simplex. The topological difference between these tessellations is that the Voronoi polyhedron describes the coordination of the nearest atomic environment while the Delaunay simplex describes the shape of the cavities between the atoms. Although the Voronoi polyhedra and the Delaunay simplexes are completely determined by each other, Voronoi polyhedra may differ topologically, while the Delaunay simplexes are always topologically equivalent and could be compared quantitatively. The Delaunay simplexes were used for structure evaluation of simple and amorphous liquids and were shown to be more adequate descriptors of the structural order of liquids then the Voronoi polyhedra. Quantitative measure of the degree of distortion of the Delaunay simplexes from the ideal tetrahedron, or 'tetrahedrality', can be calculated as:
where
is the length of the
i-th edge, and 
is the
mean length of the edges of the given simplex. In order to describe
the tetrahedrality of water one shall consider the tetrahedral
arrangement of the four nearest neighbors of any given water molecule.
These four water molecules can be treated as a quasi-Delaunay
simplex. To take into account the deviation of the central water
molecule from the geometrical center of the tetrahedron, we use:
where 
is the distance between
the oxygen of the central water molecule and the water oxygens
at the tetrahedron vertexes, and 
is the mean distance between the tetrahedron center and the vertexes.